GAZA CITY, Gaza Strip -- Thousands of Israeli soldiers invaded the Gaza Strip on Thursday, a military spokesman said, escalating a 10-day military operation to try to destroy Hamas' weapons arsenal, rocket-firing abilities and tunnels under the Palestinian territory's border with Israel.

It was the first major Israeli ground offensive in Gaza in just over five years and came after Egyptian-led efforts to broker a cease-fire failed. Earlier Thursday, 13 Hamas militants tried to infiltrate Israel through a tunnel under the Gaza-Israel border, but were stopped by an Israeli strike at the mouth of the tunnel.

The military said the operation was open-ended and would be carried out on several fronts in the coastal strip.

"Our forces, large ground forces accompanied by massive air force support, naval forces and intelligence, are taking over targets in Gaza, operating against tunnels and terror activists and infrastructure," said Israel's chief military spokesman, Brig. Gen. Motti Almoz.

He called on Gaza residents to evacuate areas where the military is operating, warning the "military is operating there with very great force."

Thousands of soldiers backed by tanks and huge DC9 bulldozers entered the Hamas-ruled territory after 10 p.m. Jerusalem time on Thursday, Israeli spokesman Lt. Col. Peter Lerner said.

Hamas spokesman Fawzi Barhoum said Israel "will pay dearly" for the assault. "Hamas is ready for a confrontation," he said.

An official in the Gaza security operations room reported heavy Israeli tank shelling along Gaza's border areas with Israel since 8 p.m. Thursday.

"All border areas are under fire. There is a tank shell every minute," said the official, speaking on condition of anonymity in line with briefing regulations. "There is also fire from the sea toward police checkpoints."

Palestinian residents and journalists in Gaza also reported heavy artillery fire from ground troops in the north and from Israeli naval gunboats stationed near Gaza's port, as well as a continuing air assault. Residents in the northern Gaza Strip said they could hear the sound of tanks entering from Beit Lahiya.

The Israeli strikes hit a range of targets, including a rehabilitation hospital. At the same time, scores of rockets from Gaza continued to stream into cities all over central and southern Israel.

Gaza news outlets reported that electricity had been cut to 80 percent of the coastal territory after cables sending power from Israel were damaged. The Palestinian death toll exceeded 240, many of them civilians; one Israeli has been killed.

Israel accuses Hamas of firing from within populated neighborhoods and using civilians as "human shields."

Jordan, the Arab representative on the United Nations Security Council, called Thursday night for an emergency meeting of the council but no time was set.

The hostilities quickly resumed at the end of a five-hour humanitarian window both sides had agreed to early Thursday to allow Gazans to stock up on food and other necessities after being largely holed up at home since the conflict began.

Thousands of Israeli soldiers had massed on the border with Gaza in recent days, waiting for the order to go in.

Israel had called up 48,000 reserve soldiers, and later Thursday the Cabinet authorized the military to call up 18,000 more, the military said.

Since July 8, Israeli strikes have hit more than 2,000 targets in Gaza and Hamas launched nearly 1,500 rockets at Israel, the Israeli military has said.

Israeli Prime Minister Benjamin Netanyahu's office said he instructed the army to go ahead with the ground offensive after Hamas rejected an Egyptian cease-fire plan earlier in the week and after Hamas militants tried to infiltrate Israel through a tunnel from Gaza.

"In light of the despicable and relentless aggression by Hamas and the dangerous infiltration into Israel, Israel is obliged to protect its citizens," the statement said.

Lerner said the next stage of Israel's Operation Protective Edge would penetrate the north, east and south of Gaza with ground forces. He said it was not aimed at toppling Hamas from its longtime rule of Gaza, but would "make sure that the Hamas terrorists will be pursued, paralyzed and threatened" by the force of the Israel Defense Forces.

"We will be striking the infrastructure, we will be striking the operatives, in order to safeguard the citizens of Israel," Lerner said. "The IDF is not bound by a time frame. We are operating to implement our goals."

Lerner said the ground forces in Gaza would include infantry, artillery, armored corps and engineering corps, supported by Israel's "vast intelligence capabilities" and aerial and naval bombardment.

Asked what Gaza residents should do to stay safe during the invasion, Lerner said: "Refrain and keep away from Hamas terrorists. Keep away from the infrastructure which is being used against the state of Israel. That's the best advice we can give them."

The United Nations said Thursday that it had discovered 20 rockets hidden in a vacant school in Gaza during a regular inspection Wednesday. The U.N. Relief and Works Agency, which assists Palestinian refugees, called the use of a school to hide weaponry a "flagrant violation" of international law protecting civilians.

Agency officials notified Palestinian and Israeli authorities of the discovery and took steps to remove the rockets. The agency said it was beginning a full investigation.

Also on Thursday, the Israeli authorities formally indicted three Israelis suspected of killing a 16-year-old Palestinian boy in Jerusalem. Officials said they believed that the killing was carried out to avenge the abduction and murder of three young Israelis last month.

Cease-fire talks

Earlier on Thursday, Palestinian, Egyptian, Israeli and U.S. officials said intense discussions were underway on terms for a cease-fire, but none was willing to be quoted by name.

"The effort to achieve an end of the violence is ongoing," said one senior Israeli official. "We're not there yet."

The U.S. has been pushing plans for the cease-fire that Egypt proposed, and Secretary of State John Kerry spoke Thursday to top diplomats in Cairo and the Qatari capital Doha to try to negotiate an end to hostilities that entered their 11th day. Kerry spoke with Netanyahu on Wednesday, U.S. State Department spokesman Jen Psaki said.

Egyptian President Abdel-Fattah el-Sissi met in Cairo Thursday with Western-backed Palestinian President Mahmoud Abbas. A day earlier, Abbas met with the deputy leader of Hamas, Moussa Abu Marzouk.

A Palestinian official in Cairo said Marzouk presented Hamas' demands for the cease-fire, which were also delivered to Jordan and the U.N. They included a desire that countries other than Egypt -- believed to be Qatar and Turkey -- be involved in forging an agreement to end the fighting, a sign of Hamas' mistrust of Cairo.

Relations between Egypt and Hamas have become fraught with tension after the ouster last year of President Mohammed Morsi, who hails from the Islamist Muslim Brotherhood group, of which Hamas is the Palestinian chapter.

Egypt's foreign minister said Thursday before the humanitarian pause that his country's proposal for a cease-fire between Israel and Hamas was gaining momentum, calling it the only viable way to stop an "intolerable humanitarian situation" in Gaza.

He also expressed frustration that "Palestinian factions" did not share what he described as Egypt's "desire ... to protect the Palestinian people in Gaza" by agreeing to the initiative.

"The only way to protect the people and to avoid additional bloodshed is acceptance of the plan," Sameh Shukri said. "The plan has been widely endorsed by the Arab League, it has been endorsed by the European Union ... and it continues to gain momentum in terms of recognition."

The plan, announced Monday, requires "unconditional acceptance" of the cease-fire by both sides, to be followed by "unfettered access" to Gaza for humanitarian aid, as well as further talks in Cairo, Shukri said.

The plan was accepted by Israel this week but rejected by Hamas, which said it had never been consulted, a claim Shukri denied.

Also Thursday, the Obama administration criticized Israel for failing to do all it can to prevent civilian casualties in Gaza during cross-border attacks.

The State Department said the high civilian death toll in Gaza has been "heartbreaking." Three more children were killed in Gaza on Thursday.

Psaki also criticized Hamas militants in Gaza who continue to fire rockets and mortars into Israel.

"I don't think we've made any secret about our strong concern about the actions of Hamas, the indiscriminate rocket attacks, the targeting of civilians," Psaki said. "And that concern remains."

Of an Israeli attack that killed four Palestinian children on Wednesday, Psaki called the attack "horrifying."

"The tragic event makes clear that Israel must take every possible step to meet its standards for protecting civilians from being killed," Psaki said. "We will continue to underscore that point to Israel."

Asked if the U.S. believes Israel has not done enough to prevent civilian casualties, Psaki said: "We believe that certainly there's more that can be done."

Information for this article was contributed by Karin Laub, Ian Deitch, Ian Phillips and Maggie Michael of The Associated Press and by Jodi Rudoren, Anne Barnard, Isabel Kershner, Fares Akram, Gabby Sobelman and Irit Pazner Garshowitz of The New York Times.

A Section on 07/18/2014