WASHINGTON - Delaying what had appeared to be an imminent strike, President Barack Obama abruptly announced Saturday that he will seek congressional approval before launching any military action meant to punish Syria for its reported use of chemical weapons in an attack that killed hundreds.

With Navy ships on standby in the Mediterranean Sea ready to launch their cruise missiles, Obama said he had decided that the United States should take military action, and he believes that as commander in chief he has “the authority to carry out this military action without specific congressional authorization.”

“I’m prepared to give that order,” Obama said. “But having made my decision as commander in chief based on what I am convinced is our national security interest, I’m also mindful that I’m president of the world’s oldest constitutional democracy.

“I know that the country will be stronger if we take this course, and our actions will be even more effective,” he said. His remarks were televised live in the United States as well as on Syrian state television.

By Saturday evening, the White House had sent Congress its draft of a resolution to authorize Obama to use military force. The draft does not lay out a specific timeline or course of military action but gives Obama approval to use the military as he determines is “necessary and appropriate” to meet its objective of preventing further chemical attacks. It also affirms the administration’s view that, ultimately, only a negotiated political settlement can resolve the crisis in Syria.

Congress is to return from a summer vacation Sept. 9, and in anticipation of the coming debate, Obama challenged lawmakers to consider “what message will we send if a dictator can gas hundreds of children to death in plain sight and pay no price.”

The president didn’t say so, but his strategy carries risks to his and the nation’s credibility, which the administration has argued forcefully is on the line in Syria. Obama long ago said the use of chemical weapons was a “red line” that Syrian President Bashar Assad would not be allowed to cross with impunity.

Officials said Obama is likely to win support in the Senate, where leading Republicans quickly issued statements welcoming his decision, but the House is more of an open question, given a strong anti-war sentiment on both sides of the aisle.

The White House wouldn’t say what options would still be open to the president if he failed to get the backing of the House and Senate.

Last week, British Prime Minister David Cameron suffered a defeat when the House of Commons refused to support his call for military action against Syria.

By accident or design, the new timetable gives time for United Nations inspectors to receive lab results from the samples they took during four days in Damascus and to compile a final report. After leaving Syria on Friday night, the inspection team arrived in Rotterdam, Netherlands, a few hours before Obama spoke.

The group’s leader was expected to brief U.N. Secretary-General Ban Ki-moon today.

The inspection team is determining whether a chemical attack occurred, but not who ordered it and carried it out. The report may take weeks to prepare because of the lab work required, said a U.N. official who wasn’t authorized to comment and asked not to be identified.

The U.N. needs time to examine the “entire body of evidence” and will produce an “impartial, credible” report on the chemical-weapons allegations, said Martin Nesirky, spokesman for the secretary-general.

Nesirky said the departure of the U.N. inspection team should not be seen as any kind of a green light for military action.

“I’ve seen all kinds of reporting suggesting that the departure of the chemical-weapons team somehow opens a window for military action of some kind,” Nesirky said. “Frankly, that is grotesque, and it’s an affront to the more than 1,000 staff, U.N. staff, who are on the ground in Syria delivering humanitarian aid and who will continue to deliver critical aid.”

A four-page intelligence assessment released by the Obama administration Friday didn’t tie the attack directly to orders from Assad, though it said he’s Syria’s ultimate decision-maker. Administration officials have previously said the Syrian president bears responsibility for his military’s actions.

The report concluded with “high confidence” that the Assad government carried out the attack. Syria has a stockpile of chemical agents and “thousands” of munitions to deliver them, it said.

Administration officials said Obama appeared to be set on ordering a strike until Friday evening. After a walk around the White House grounds with Chief of Staff Denis McDonough, the president told his aide that he had changed his mind.

Officials said Obama told McDonough that he wanted political accountability - lawmakers from both parties, he believed, should be on the record in support or against the war. And unlike the U.S.-led military operation in Libya in 2011 - which was supported by the U.N. Security Council and the Arab League - Obama told McDonough that the U.S. did not have the same level of international backing.

Officials said Obama initially drew resistance in a two-hour session attended by Vice President Joe Biden, Defense Secretary Chuck Hagel, Director of National Intelligence James Klapper, CIA Director John Brennan, national security adviser Susan Rice and homeland security adviser Lisa Monaco.

Obama had several reasons for his decision, including a sense of isolation after the setback in the British Parliament, aides said. But the most compelling one may have been that acting alone would undercut him if in the next three years he needed congressional authority for his next military confrontation in the Middle East, perhaps with Iran.

Had he gone ahead with a military strike, Obama would have become the first U.S. leader in three decades to attack a foreign nation without mustering broad international support or acting in direct defense of Americans. Not since 1983, when President Ronald Reagan ordered an invasion of the Caribbean island of Grenada, has the U.S. been so alone in pursuing major military action beyond a few attacks responding to strikes or threats against its citizens.

Republicans generally expressed satisfaction with Obama’s decision to seek congressional support, and challenged him to make his case to the public and lawmakers that American power should be used to punish Assad.

“We are glad the president is seeking authorization for any military action in Syria in response to serious, substantive questions being raised,” House Republican leaders said in a joint statement.

Obama spoke with the Democratic and Republican leaders of both houses before his announcement and said they agreed to schedule a debate and votes. Public hearings before the Senate Foreign Relations Committee and classified and unclassified briefings for senators were being scheduled for this week, Senate Majority Leader Harry Reid, D-Nev., said Saturday. Both the Senate and the House planned a vote on the matter no later than the week of Sept. 9.

Sen. Mitch McConnell, the Senate Republican minority leader, welcomed Obama’s decision.

“The president’s role as commander in chief is always strengthened when he enjoys the expressed support of the Congress,” McConnell said.

The administration’s effort at persuasion is already well underway. The White House assigned Secretary of State John Kerry, Hagel and other top officials to brief both parties in the Senate by telephone Saturday and scheduled a classified briefing in person on Capitol Hill today for any lawmakers in town.

Obama also challenged lawmakers to consider “what message will we send to a dictator” if he is allowed to kill hundreds of children with chemical weapons without suffering any retaliation.

At the same time, a senior State Department official said Kerry spoke with Syrian Opposition Coalition President Ahmed Assi al-Jarba to underscore Obama’s commitment to holding the Assad government accountable for the Aug. 21 attack.

On Saturday, French President Francois Hollande’s office said it will wait for its parliament and the U.S. Congress to consider possible military action before making a decision about whether to launch strikes.

Obama said Friday that he was considering “limited and narrow” steps to punish Assad, adding that U.S. national security interests were at stake. He pledged no U.S. combat troops on the ground in Syria, where a civil war has claimed more than 100,000 civilian lives.

Syrians opposed to Assad expressed anger Saturday that Obama would not imminently authorize military action, saying that failure to act would embolden the Assad government.

“Assad has been given the green light by the international community,” said Abu Qatada of the Damascus Military Council. “The message he got from the international community is that he can kill his people with conventional means, just not with chemical weapons.”

Before Obama’s announcement, Syrians in Damascus braced for a possible attack by stocking up on food, including staples such as bread, rice and water. Traffic on the streets, which are usually crowded in the summer evenings, was sparse Friday night.

“I watch talk shows and listen to analysts to try to understand what’s going on and guess what will happen,” said Adel Mustapha, 52, who works for a travel company. “Attacking Syria will be tantamount to terrorism.”

With Obama struggling to gain international backing for a strike, Russian President Vladimir Putin urged him to reconsider his plans. Putin, saying he was appealing to a Nobel Peace laureate rather than to a president, urged Obama to avoid military action.

“We have to remember what has happened in the last decades, how many times the United States has been the initiator of armed conflict in different regions of the world,” said Putin, an Assad ally. “Did this resolve even one problem?”

The Russian leader said he was convinced that the suspected chemical attack was a provocation aimed at drawing the U.S. military into Syria’s civil war, implying the he believes the attack was carried out by the Syrian rebels.

If the Americans have evidence proving the involvement of Assad’s regime, they should present it to the U.N. inspectors and the Security Council, he said. “If there is evidence, it should be presented,” Putin said. “If it is not presented, that means it does not exist.”

Even the administration’s casualty estimate was grist for controversy, as a group that monitors casualties in the Syrian civil war challenged the U.S. to substantiate its claim that 1,429 died in a chemical weapons attack, including more than 400 children.

The Syrian Observatory for Human Rights, an organization that monitors casualties in the country, said it has confirmed 502 deaths, nearly 1,000 fewer than the American intelligence assessment claimed.

Rami Abdel-Rahman, the head of the organization, said he was not contacted by U.S. officials about his efforts to collect information about the death toll in the Aug. 21 attacks.

Outside the gates of the White House on Saturday, the chants of protesters could be heard as the president stepped to a podium in the Rose Garden.

Almost 80 percent of Americans said Obama should seek congressional approval before taking any military action, according to a poll conducted last week for NBC News. Only 42 percent said they would support a U.S. military response, rising to 50 percent when the action specified is limited cruise-missile strikes targeting Syrian infrastructure used to carry out chemical-weapons attacks. The poll of 700 adults has an error margin of 3.7 percentage points.

In Little Rock, more than two dozen protesters gathered at the Arkansas Capitol to oppose a possible U.S. attack. Some wore T-shirts proclaiming “No U.S. intervention in Syria.”

“I had friends that died in Iraq, and I don’t want more people to die for nothing,” said Dominic Box, 23.

Information for this article was contributed by David Espo, Julie Pace, Josh Lederman, Matthew Lee, Kimberly Dozier, Zeina Karam, Yasmine Saker, Karin Laub, Geir Mouslon, Juan A. Lozano, Ramit Plushnick-Masti, Steve LeBlanc, Rodrique Ngowi, Jeannie Nuss, Sara Burnett, Geir Moulson, Sylvia Hui, Eric Tucker, Laurie Kellman, Jamey Keaten, Sylvie Corbet, Sarah El Deeb, Jamal Halaby, Peter James Spielmann and Lynn Berry of The Associated Press; by Peter Baker and Mark Landler of The New York Times; by Margaret Talev, Terry Atlas, Donna Abu-Nasr, Dana El Baltaji, Mark Deen, Robert Tuttle, Gopal Ratnam, Indira A.R. Lakshmanan, Nicole Gaouette, Tony Capaccio, David Lerman, John Walcott, Timothy R. Homan, Heidi Przybyla, Roger Runningen, Dave Michaels, Roxana Tiron, Sangwon Yoon, Ksenia Galouchko, Ilya Arkhipov and Eddie Buckle of Bloomberg News; and by Liz Sly, Ahmed Ramadan, Loveday Morris and Scott Wilson of The Washington Post.

Front Section, Pages 1 on 09/01/2013