Smoke rises after an Israeli missile strike in the Gaza strip as seen from the Israel Gaza border on Wednesday, July 16, 2014.

GAZA CITY, Gaza Strip - Israel agreed to implement a five-hour "humanitarian" pause in its 9-day-old bombardment of Gaza that has killed more than 200 Palestinians, including four boys struck on a beach Wednesday by shells fired from a navy ship.

Israel said it would hold its fire Thursday from 10 a.m. (0700 GMT, 3 a.m. EDT) under a U.N.-brokered plan to allow Palestinians to restock supplies. But it vowed to retaliate "firmly and decisively" if Hamas or other militant groups launch attacks on Israel during that time.

Robert Serry, the U.N. special coordinator for the Middle East peace process, had asked Israel to agree to a "unilateral humanitarian pause" so that food, water and other necessities can be delivered to Gaza, said U.N. deputy spokesman Farhan Haq. Serry will "urge the parties in Gaza to respect that pause," Haq said.

There was no word on whether there would be a similar lull from Palestinian militants, who fired at least 90 rockets at Israel on Wednesday and vowed not to stop until their demands were met.

Israel's military said its forces bombed at least 150 targets in Gaza on Wednesday. It did not provide more specifics, but the Gaza Interior Ministry's website said 30 houses, including those of senior Hamas leaders Mahmoud Zahar, Jamila Shanti, Fathi Hamas and Ismail Ashkar, were targeted.

Zahar was a key figure in Hamas' violent takeover of Gaza in 2007, while the other three were members of the Palestinian parliament elected in 2006. Many Hamas leaders have gone into hiding since the beginning of the Israeli offensive.

Palestinian President Mahmoud Abbas met in Cairo with a senior official from the Hamas militant group to try to salvage an Egyptian cease-fire proposal on ending the conflict.

The Egyptian plan had collapsed shortly after it was announced late Monday. Hamas official Sami Abu Zuhri said Wednesday his group had issued its formal rejection of the plan, and he bemoaned what he called little support from the Arab world.

Gaza health official Ashraf al-Kidra said the Palestinian death toll rose to 222, with 1,670 wounded. Only one Israeli has so far died in the conflict - a civilian distributing food to soldiers in southern Israel on Tuesday evening - largely because of the effectiveness of Israel's Iron Dome air defense system in downing incoming rockets.

The four boys, who were cousins aged 9 to 11, were killed on the beach beside a coastal road west of Gaza City, al-Kidra said. Seven others - adults and children - were wounded in the strike, which Palestinian human rights activist Khalil Abu Shamalla and Palestinian health officials said came from an Israeli naval vessel.

A witness who identified himself only as Abu Ahmed said the boys were scavenging for scrap metal when a first shell hit a nearby shipping container used in the past by Hamas security forces. He said the boys fled but a second rocket "hit all of them."

The Israeli army said in a statement it was "carefully investigating" the matter. It said the target of the naval attack was "Hamas terrorist operatives" and that civilian casualties were "a tragic outcome."

It said the army "has no intention of harming civilians dragged by Hamas into the reality of urban combat."

The boys' uncle, Abdel Kareem Baker, 41, raged at Israel after the attack.

"It's a cold-blooded massacre," he said. "It's a shame who come they didn't identify them as kids with all of the advanced technology they claim they're using."

Israel's security Cabinet approved a call-up of 8,000 additional reservists, according to an Israeli official, also speaking on condition of anonymity because he was not permitted to discuss the matter publicly.

The army said 45,000 reservists have already been summoned - a sign that Israel was ramping up its threat to carry out a ground invasion, although experts have said such an action was highly unlikely.

Abbas met with deputy Hamas chief Moussa Abu Marzouk to discuss the Egyptian initiative. An official with Abbas' Fatah faction told The Associated Press that Abbas and Egypt were trying to get Hamas on board.

Egypt's ambitious cease-fire plan seeks to restore greater authority for Abbas in Gaza and settle rifts between his Palestinian Authority and Hamas, which took control of the seaside territory in 2007, according to a Palestinian official in Cairo. The official spoke on condition of anonymity because he wasn't authorized to release information about the closed-door talks.

Under the proposal, a cease-fire would be followed by talks to resolve the rift between Hamas and Abbas over Gaza, including giving control of the border to Abbas, he said. That would presumably allow a greater opening of the borders, one of Hamas' main demands, along with the easing of border closures and the release of former Hamas prisoners who had been freed by Israel in a 2011 prisoner exchange but were rearrested by Israel last month in the West Bank.

The official, who was in Cairo, said Hamas' main objection was the power-sharing arrangement.

Israeli Prime Minister Benjamin Netanyahu has said Hamas would pay a high price for rejecting the Egyptian plan.

Al-Kidra, the Gaza health official, said an airstrike in the Gaza neighborhood of Khan Younis killed four members of the Al-Astal family, including a 6-year-old boy, his 4-year-old sister, and a 70-year-old woman.

In a separate airstrike, a woman and a young girl were killed, al-Kidra said.

Israel told tens of thousands of residents of the northern town of Beit Lahiya and the Zeitoun and Shijaiyah neighborhoods of Gaza City, all near the border with Israel, to evacuate their homes by Wednesday morning. The Israeli military said in automated phone calls, text messages and air-dropped leaflets that large numbers of rockets were launched from these areas and that it planned to bomb these locations.

"Whoever disregards these instructions and fails to evacuate immediately, endangers their own lives, as well as those of their families," the message said.

As a result, hundreds of residents of Zeitoun and Shijaiyah were seen walking in the streets, carrying small bags with belongings.