GAZA CITY (CNN) -

A brief respite from the violence in Gaza appeared imminent Wednesday after both sides agreed to a U.N-requested temporary cease-fire.

"Factions of the resistance have agreed to accept the offer of the UN regarding a 'field calm' for 5 hours from 10 AM until 3 PM (local time) Thursday for humanitarian needs," Hamas spokesman Sami Abu Zhuri said in a text message.

Israel had already accepted the proposal; however, the military warned it would not sit idle if attacked.

"Should the humanitarian window be exploited by Hamas or other terror organizations for the purpose of launching attacks against Israeli civilian or military targets the IDF will respond firmly and decisively," the Israel Defense Forces said in a statement.

An effort to permanently stop the killing stalled Tuesday when Israel resumed airstrikes following a brief, one-sided cease-fire brokered by Egypt. While Israel paused for six hours, Hamas leaders rejected the deal and continued firing rockets.

They said they had not been consulted, and complained the deal did not address their demands for greater freedom for Gaza's 1.8 million residents.

"The initiative is no longer acceptable, and there is no basis for the continuation of this initiative," Hamas spokesman Zhuri had told CNN.

Children on beach killed

Anger is rising over civilian deaths -- including those of four children killed while playing on the beach.

The boys, ages 9 to 11, died Wednesday when a shell from an Israeli gunship exploded near them at the Al-Shati refugee camp in northern Gaza, according to Palestinian officials.

An Israeli official said the shelling was another example of Hamas using civilians as human shields -- intimating that the boys had been left to play near a rocket launcher.

"What they are deliberately doing is seeking to kill as many Palestinians as possible in order to yell to the world to, 'Help us,' " Israeli Cabinet member Naftali Bennett told CNN. "This is cynical and this is cowardly."

A Hamas official, however, called the shelling by an Israeli gunship a "massacre" and a "war crime" and demanded U.N. action.

"Those children were not firing rockets, they were just playing," Hamas spokesman Zhuri told reporters.

Israel's military said it was aware of the report and was investigating. The military, spokesman Brig. Gen. Moti Almoz said, never intentionally targets civilians.

"Based on preliminary results, the target of this strike was Hamas terrorist operatives. The reported civilian causalities from this strike are a tragic outcome," the IDF said in a statement.

"We're checking it out because we don't target civilians. We don't target children. There's obviously been a mistake," Israeli government spokesman Mark Regev told CNN's "The Situation Room with Wolf Blitzer."

The incident inflamed already raw emotions over civilian deaths in Gaza, where at least 221 Palestinians have died and close to 1,600 have been injured since Israel began its anti-Hamas military operation July 7, according to Palestinian health officials.

The one fatality on the Israeli side so far occurred Monday when a mortar shell hit a man at the Erez border crossing, Israeli Rescue Services said.

Rejection of Egyptian efforts

Also Wednesday, Hamas declined to join talks in Cairo between Egyptian President Abdel Fattah el-Sisi and Palestinian Authority President Mahmoud Abbas.

Like Israel, Egypt considers Hamas a terror organization because of the group's roots in the Muslim Brotherhood, which Egypt's military-led government banned after the country's 2013 coup.

Hamas officials said they are not opposed to a cease-fire, but want to see a broad agreement that would, among other things, end restrictions on border crossings that they say are choking the life out of Gaza's 1.8 million residents.

"I think what they want is to see a cease-fire agreement that addresses the real problems that they face and the system of violence that is this siege, that is the occupation, so that it can be a genuine cease-fire agreement that brings an end to hostilities, not just from one side," Yousef Munayyer, executive director of the Palestine Center, a pro-Palestinian think tank, told CNN's "New Day."

But Israeli Prime Minister Benjamin Netanyahu said that Hamas had brought the continued Israeli operation on itself after rejecting the Egyptian-brokered cease-fire.

"We held our fire for six hours and during that time, Hamas continued to barrage our cities with rockets," Netanyahu said. "Hamas thus shut the door to a diplomatic solution, and it therefore bears sole the responsibility for the continuation of the violence."

Former Palestinian Authority Foreign Minister Nabil Shaath said Hamas has legitimate complaints about how Israel has implemented past accords.

"Gaza is still fully under siege. And none of the agreements that have been signed before have been implemented," he told CNN's Blitzer. "But we think this is something that can be discussed later. What we should do now is proceed to an immediate stop of the Israeli attack on Gaza and, therefore, an immediate cease-fire. And we are working very hard to make that happen as soon as possible."

'We are in jail here'

Both Palestinians and Israelis say they are living in fear.