Israel resumed its deadly bombardment of Gaza on Friday despite international calls for a renewed truce, killing at least 178 Palestinians and wounding more than 589 people.
Clouds of grey smoke from strikes shot up on the Gaza skyline and rockets fired from the territory streaked into Israel as Israel resumed its relentless offensive.
The health ministry in Gaza said at least 178 people had been killed there since the pause in hostilities expired early on Friday.
UN agencies warned of a catastrophic humanitarian situation in Gaza as hospitals again struggled to cope with the wounded after a week-long respite.
Israel alleged that Hamas attempted to break the truce before it ended at 0500 GMT by firing a rocket and that it failed to produce a list of further hostages for release on Friday after seven earlier exchanges.
As the bombardment resumed, Hamas's armed wing received "the order to resume combat" and to "defend the Gaza Strip", according to a source close to the group.
United Nations chief Antonio Guterres and the White House called for the break in fighting to be restored.
"We continue to work with Israel, Egypt and Qatar on efforts to extend the humanitarian pause in Gaza," a US National Security Council spokesperson said.
Seven days of hostage-prisoner exchanges had yielded tearful reunions of Israeli families with their released relatives and jubilation in the streets of the occupied West Bank as some Palestinian women and child prisoners walked free from Israeli jails.
US Secretary of State Antony Blinken told reporters in Dubai that the United States remained "intensely focused on getting everyone home, getting hostages back" and "pursuing the process that had worked for seven days" during the truce.
But Israeli government spokesman Eylon Levy told reporters: "Having chosen to hold onto our women, Hamas will now take the mother of all thumpings."
The Israeli forces said that "ground, air and naval forces struck terror targets in the north and south of the Gaza Strip, including in Khan Yunis and Rafah."
Outside the Al-Ahli hospital in Gaza City, a man in a blue sweater bellowed in grief and turned his face and hands to the sky after viewing a dead boy in a body bag, AFPTV footage showed.
"What did he do wrong? God, what did we do to deserve this?" he cried.
During the unprecedented October 7 attack, Hamas fighters broke through Gaza's heavily militarised border into Israel, resulting in the killing of around 1,200 Israelis and taking around 240 captives, according to Israel.
Israel unleashed a relentless air and ground campaign that killed more than 15,000 people, mostly civilians - including some 8,000 children.
Guterres has warned of a "humanitarian catastrophe" in Gaza, where the United Nations says 1.7 million people are displaced and short of food, water and other essentials.
"The healthcare service is on its knees," Rob Holden, a World Health Organization (WHO) senior emergency officer, told journalists in Geneva on a video-link from Gaza as explosions were heard in the background.
"It is like a horror movie."
On a bed at Khan Yunis's Nasser hospital, Amal Abu Dagga wept, her beige veil covered in blood.
"I don't even know what happened to my children," she said. A relative, Jamil Abu Dagga, told AFP the family had been at home when the bombs started falling.
Mediation efforts by Qatar and Egypt were "ongoing", said a source briefed on the talks who asked not to be named.
During the seven-day truce, Hamas freed 80 Israeli captives in exchange for 240 Palestinian women and child prisoners, and more aid entered Gaza.
Twenty-five other hostages, mostly Thais, were also freed in separate arrangements.
On Thursday Blinken, meeting Israeli and Palestinian officials, called for the truce to be extended, and warned any resumption of combat must protect Palestinian civilians.
Residents in various areas of Gaza were sent SMS warnings on Friday.
Israeli forces "will begin a crushing military attack on your area of residence with the aim of eliminating the terrorist organisation Hamas," the warnings said.
Also in Lebanon, Israeli bombardment killed three people on the border, hours after the truce collapsed on Friday.