WASHINGTON — The State Department on Thursday acknowledged major weaknesses in security and errors in judgment exposed in a scathing independent report on the deadly Sept. 11 assault on a U.S. diplomatic mission in Libya. Two top State officials appealed to Congress to fully fund requests to ensure that diplomats and embassies are safe.

Testifying before two congressional committees, senior State Department officials admitted that serious management and leadership failures left the diplomatic mission in Benghazi woefully unprepared for the terrorist attack that killed Ambassador Chris Stevens and three other Americans.

“We clearly fell down on the job with regard to Benghazi,” Deputy Secretary of State William Burns told the House Foreign Affairs Committee. Earlier, before the Senate Foreign Relations Committee, Burns said: “We learned some very hard and painful lessons in Benghazi. We are already acting on them. We have to do better.”

Burns and Deputy Secretary of State Thomas Nides testified in place of Secretary of State Hillary Rodham Clinton, who was under doctor’s orders to stay home and recover from a concussion she suffered last week. Burns and Nides reiterated Clinton’s written acceptance of the panel’s report and vowed to implement each of its 29 recommendations.

The White House on Thursday also made its first comment on the findings of the report. Spokesman Jay Carney said that what happened in Benghazi was “clearly unacceptable” and that problems had to be fixed.

The report found that“systemic failures and leadership and management deficiencies at senior levels” of the State Department meant that security was “inadequate for Benghazi and grossly inadequate to deal with the attack that took place.”

The Senate hearing provided an odd scene because the committee chairman, Sen. John Kerry, D-Mass., is the top candidate to replace Clinton as secretary of state in President Barack Obama’s second-term Cabinet. Kerry presided at the hearing but asked no questions of officials who could be his future employees.

In an opening statement, Kerry said the department had “clear warning signs” of a deteriorating security situation before the attack. He also faulted Congress for failing to provide sufficient money to protect facilities worldwide, forcing the department to scramble to cover security costs.

Nides told lawmakers Thursday that the State Department will hire more security guards and use more Marines to safeguard high risk posts around the world in an effort to prevent assaults on U.S. diplomats after the deadly attack in Libya.

Responding to the review panel’s findings of “systemic failures” and “grossly inadequate” security for the U.S. mission in Benghazi, Libya, the department plans to hire 150 more diplomatic security personnel, a 5 percent increase, he said. The Pentagon has approved dispatching an additional 225 Marine Corps guards, he said.

The State Department is seeking about $1.4 billion in next year’s budget for increased security; the money would come primarily from funds that haven’t been spent in Iraq. That would include $553 million for 35 more Marine Security Guard units, $130 million for 155 diplomatic security agents and $376 million for security upgrades and construction at new embassies.

Since the attack, Democrats have complained that Republicans cut $300 million from the Obama administration’s budget request of $2.6 billion for diplomatic and embassy security this year.

But the chairman of the House Foreign Affairs Committee, Rep. Ileana Ros-Lehtinen, R-Fla., noted that “budget constraints were not a factor in the department’s failure” to recognize the threat.

Meanwhile, the volatility of Benghazi was underscored Thursday when four people were killed after militants attacked a security headquarters.

The attack on the security directorate building - with rocket-propelled grenades and hand grenades, according to authorities and witnesses - was the latest in a string of attacks on police facilities since last month.

The attack Thursday appeared to be an attempt by militants to free suspects held by the police in connection with the death of the security director, Faraj Mohammed al-Drissi, and other assassinations.

Two militiamen, a national guard soldier and a police officer were killed in Thursday’s attack, according to local officials.

The State Department’s diplomatic security budget increased from about $200 million in 1998 to $1.8 billion in 2008, according to the Government Accountability Office. Much of that has gone into physical upgrades at embassies and consulates to meet specifications adopted after the deadly 1998 bombings of the U.S. embassies in Kenya and Tanzania.

Nides noted that money for construction has been significantly reduced so that the number of new compounds being built has dropped from the goal of 10 per year to two.

Republicans tangled with Burns and Nides over whether warning signs of a deteriorating security situation were ignored and why the department didn’t ask Congress for money to boost security at the mission in the eastern Libyan city that was left relatively lawless after the revolution that toppled strongman Moammar Gadhafi.

The fallout from the Benghazi attack and the initial explanation offered by the U.S. ambassador to the United Nations, Susan Rice, that it was the result of a protest against an American-made, anti-Islam video has become deeply partisan. Republicans accused the administration of trying to cover up a terrorist incident to help President Barack Obama’s re-election bid.

Stevens was killed in the attack along with information specialist Sean Smith and former Navy SEALs Glen Doherty and Tyrone Woods, who were contractors working for the CIA. Stevens was the first U.S. ambassador killed in the line of duty since 1979.

The conclusions of the Accountability Review Board that Clinton convened led to the resignation or reassignment of four State Department officials who worked in two bureaus - Diplomatic Security and Near East Affairs - that were singled out for the harshest criticism in the report.

The resignations did little to mollify lawmakers who insisted that Clinton testify in the coming weeks despite her plan to leave the administration. Kerry said she would appear before the panel in January.

“She is ultimately responsible for the department and U.S. posts around the world. Her testimony before the Senate Foreign Relations Committee is indispensable to any effort to address this failure and put in place a process to ensure this never happens again,” said Sen. Marco Rubio, R-Fla.

Information for this article was contributed by Matthew Lee, Kimberly Dozier, Donna Cassata and Richard Lardner of The Associated Press; by Osama al-Fitory and Kareem Fahim of The New York Times; and by Laura Litvan and David Lerman of Bloomberg News.

Front Section, Pages 1 on 12/21/2012