Hamas militants and Israel exchanged rocket fire Thursday following the expiration of a brief cease-fire that allowed Gazans to stock up on supplies. 

The resumption of violence signaled major obstacles to Egyptian-led efforts to reach a permanent truce between the two sides, despite increased efforts by Egypt to broker a deal.

The U.N.-brokered five-hour lull in fighting gave residents of the Gaza Strip time to crowd into stores and vegetable markets after more than a week of being mostly holed up at home for fear of airstrikes. Gaza City, a virtual ghost town for the past 10 days, returned to apparent normalcy within minutes of the start of the truce. Streets were jammed, motorists honked horns and Hamas police directed traffic at busy intersections.

But the streets emptied out quickly after the cease-fire expired, with Palestinian militants firing more than 50 rockets at Israel, including a heavy salvo toward the Tel Aviv area that sent people running for cover, the Israeli military said. The military said a rocket fired from Gaza struck the city of Ashkelon at precisely 3 p.m. local time (8 a.m. ET) as the pause in military activity ended. No injuries were reported.

Israel responded with a wave of eight airstrikes, including one that killed two boys and a girl ages 8 to 10 from the same family in Gaza City, Gaza Health Ministry spokesman Ashraf al-Kidra said.

TV footage from the scene showed a doll and a sandal near pools of blood on the roof of the home.

The deaths came a day after four boys ages 9 to 11 were killed on the beach beside a coastal road west of Gaza City. Israel issued a renewed warning Thursday to Gaza residents to leave their homes for their own safety.

Mounting civilian casualties have increased international pressure to stop the hostilities, but negotiators have made little headway.

Israel accuses Hamas of firing from within populated neighborhoods, using civilians as "human shields" and mosques, homes and schools for storing weapons.

On Thursday, the U.N. refugee agency for Palestinians, UNRWA, said that during a routine check it discovered about 20 rockets hidden in one of its vacant Gaza schools and called on militants to respect the "sanctity and integrity" of U.N. property.

"This incident, which is the first of its kind in Gaza, endangered civilians including staff and put at risk UNRWA's vital mission to assist and protect Palestine refugees in Gaza," the agency said.

The rockets were removed, Sky News reported.

Shortly before the cease-fire took effect, Israel said it had thwarted an attempted attack by 13 Islamic militants.

Israeli military spokesman Lt. Col. Peter Lerner told The Associated Press that the would-be attackers attempted to sneak into Israel through a tunnel from Gaza. They were spotted at the tunnel's opening approximately 820 feet inside Israel, near a kibbutz, and were struck by Israeli aircraft. Lerner said the military believed at least one militant was killed in the strike and that the remaining fighters appeared to have returned to Gaza through the tunnel.

Lerner said the attack "could have had devastating consequences" and said the militants were armed with "extensive weapons," including rocket-propelled grenades.

The attack was preceded by a volley of 15 rockets fired from Gaza into central Israel. The Times of Israel reported that IDF spokesman Brig. Gen. Moti Almoz told Israeli television Channel 2 that the rockets were meant to be "an envelope for this attack."

"We knew this would come," Almoz said. "We knew specifically about this tunnel. We knew Hamas would try [to launch a terror attack] in any way it can."

Neither Hamas nor other Palestinian militant groups immediately claimed responsibility for the attack. Lerner said that the incursion had not affected Israel's plan to support the truce.

However, Almoz told The Times of Israel that the Israeli Defense Forces would not hesitate to launch new attacks to prevent rocket fire by Hamas, adding that the five-hour period was a "humanitarian window" to help "the population trapped in Gaza under a regime that uses it as hostages."

It was the second time militants attempted to sneak into Israel in this round of fighting. Last week, four fighters were killed when they infiltrated Israel from the sea.

Egyptian efforts to bring all sides to the table continued Thursday, with President Abdel Fattah el-Sissi meeting with Palestinian leader Mahmoud Abbas after Egyptian officials met separately with representatives of Israel and Hamas in Cairo. But the gaps remain wide.

Israel accepted Egypt's call earlier this week to halt all fighting, but Hamas rejected the idea because it first wants to lock in achievements, such as easing the seven-year blockade of Gaza by Israel and Egypt.

In Cairo, Egypt's Foreign Minister Sameh Shukri insisted in an interview with The Associated Press that the cease-fire deal was still alive and expressed frustration that "Palestinian factions" ¡ª a clear reference to Hamas ¡ª had not agreed to it.

Hamas' agreement is crucial to any such truce, but its demand that the blockade be eased significantly is likely to be rejected by Israel and Egypt because it would strengthen the group's hold on Gaza, where it seized power in 2007.

On Wednesday, the No. 2 in Hamas, Moussa Abu Marzouk, presented a list of demands to Egypt, including that Gaza's crossings be opened and all types of goods be allowed into the territory, said a senior official in the group who spoke on condition of anonymity because he was not authorized to discuss the negotiations with reporters.

Israel has allowed consumer goods into Gaza, but has restricted construction material, fearing it would be diverted for military use. Israel has also barred most exports from Gaza, crippling the local economy.

Hamas also wants to be allowed to build a sea port as a gate to the world, with shipments under international monitoring, the Hamas official said.

In addition, Hamas demands the release of 52 activists who had been released by Israel in a 2011 prisoner swap, but were rearrested in recent months.

A high-level Israeli delegation also visited Cairo for several hours on Wednesday to discuss the terms of a cease-fire, said Egyptian government officials who spoke on the condition of anonymity because they were not authorized to speak to the media.

Thursday's temporary truce came after Israel carried out nearly 2,000 air strikes on Gaza over 10 days and Hamas fired more than 1,300 rockets into Israel, reaching the country's economic and cultural heartland. The cross-border fighting has so far killed more than 230 Palestinians and an Israeli, according to officials.

However, Almoz told The Times of Israel that the Israeli Defense Forces would not hesitate to launch new attacks to prevent rocket fire by Hamas.

Also Thursday, a Jerusalem court indicted a 29-year-old and two 16-year-olds in the death of Mohammed Abu Khdeir, 16, whose charred body was found after he was reported kidnapped. According to the indictment, the suspects went out for a "man hunt" that ended when they "cruelly" killed Abu Khdeir.

The indictment said the suspects carried out the crime in revenge for the deaths of three Israeli teens last month and that they killed Abu Khdeir "solely because he was an Arab." The suspects are also accused of attempting to kidnap a seven-year-old Arab boy a day earlier.

The indictment said Abu Khdeir was strangled, beaten and burned to death while he was unconscious. Israel's Ministry of Defense recognized Abu Khdeir as a "victim of terrorism."

The death led to days of clashes between Palestinian protesters and police in east Jerusalem and elicited widespread international condemnation.