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Those in power decided the “killing of children would recommence” in Gaza, the UN said on Friday.
Israel military said it resumed its assault in the Gaza Strip minutes after a temporary truce with Hamas expired on Friday morning.
Israel and Hamas accused each other of rejecting terms to extend the Qatari-mediated agreement, which ended at 7am local time (9am UAE time), with neither side announcing a deal to extend it.
Hours later, Gaza's health ministry reported that 109 people had been killed and dozens wounded in air strikes on the strip.
The Israeli army said it had struck more than 200 targets on Friday.
"Over the last few hours, ground, air and naval forces struck terror targets in the north and south of the Gaza Strip, including in Khan Younis and Rafah," it said.
James Elder, spokesman for UN's children's agency Unicef, said “inaction, at its core, is an approval of the killing of children”.
"It is reckless to think more attacks on the people of Gaza will lead to anything other than carnage,” he told reporters in Geneva through a video link from Gaza.
Two children were killed in strikes on Gaza City, said Fadel Naim, a doctor with Al Ahli hospital.
The US will continue to press for extending a truce in Gaza, the White House said on Friday.
"We continue to work with Israel, Egypt, and Qatar on efforts to extend the humanitarian pause in Gaza," a White House National Security Council representative said.
The Wall Street Journal reported on Friday that Israel and Hamas agreed to extend the pause in fighting in the enclave. The report quoted Egyptian officials.
But Hamas's Interior Ministry said Israel launched air strikes in the south of the strip on Friday morning, with the Shehab News Agency reporting explosions and gunfire in the north. Other Hamas-affiliated media said military aircraft and drones flew over Gaza city.
In Israel, air raid sirens sounded near the Gaza border and the military said its Iron Dome missile defence system intercepted a rocket fired from the enclave.
"With the resumption of fighting we emphasise the Israeli government is committed to achieving the goals of the war – to free our hostages, to eliminate Hamas and to ensure that Gaza will never pose a threat to the residents of Israel," the office of Israeli Prime Minister Benjamin Netanyahu said.
Ezzat El Rashq, a member of the Hamas political bureau, said: "What Israel did not achieve during the 50 days before the truce it will not achieve by continuing its aggression after the truce."
UN Secretary General Antonio Guterres said he “deeply regrets" that military operations have restarted.
“I still hope that it will be possible to renew the pause that was established. The return to hostilities only shows how important it is to have a true humanitarian ceasefire,” he wrote on X.
The seven-day truce, which started on November 24, paused the conflict, which began on October 7 when Hamas militants launched an attack in southern Israel.
Rocket sirens sounded in several towns in the north of Israel near the border with Lebanon on Friday, sending residents in the area running for shelter.
The Israeli military did not immediately share details regarding what set off the sirens, which sounded hours after a Gaza truce ended
After two last-minute extensions, the two sides marked the seventh day of the agreement on Thursday with the exchange of eight hostages and 30 Palestinian detainees, while more humanitarian aid entered the strip.
Qatar and Egypt, which played key roles in mediating the truce, attempted to prolong the pause by two days.
Mark Regev, an adviser to Israeli Prime Minister Benjamin Netanyahu, said the country was open to maintaining the truce if Hamas committed to further hostage releases.
"We're ready for all possibilities. Without that, we're going back to the combat," Mr Regev told CNN.
Israel previously set the release of 10 hostages a day as the minimum it would accept to pause its assault. On Thursday, Hamas and the Palestinian Islamic Jihad militant group put their fighters on alert as a return to fighting loomed.
Israel has sworn to annihilate Hamas, which controls Gaza, for the attack last month that killed 1,200 people and led to 240 hostages being taken to the enclave. More than 15,000 people have been killed in Israeli retaliatory attacks.
