Clouds of grey smoke from the strikes hung over Gaza, where the health ministry said nearly 200 people had been killed since the pause in hostilities expired early Friday.
As Israeli war on Gaza 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.
International leaders and humanitarian groups condemned the return to Israeli bombardments.
"I deeply regret that military operations have started again in Gaza," UN secretary-general Antonio Guterres said on X, formerly Twitter.
Fears of a wider regional conflict grew after the Syrian defense ministry said Israeli strikes had hit Damascus on Saturday and the resistance group Hezbollah said one of its members had been killed in an Israeli strike on Lebanon on Friday.
The United States said it is working with regional partners to reach another ceasefire.
"We're going to continue to work with Israel and Egypt and Qatar on efforts to reimplement the pause," US Defense Secretary Lloyd Austin told reporters in California on Friday.
Israel unleashed an air and ground campaign on Gaza, on 7 October that has killed more than 15,000 people, almost two-thirds of them women and children, the Gaza authorities say.
The Israeli occupation army said on Friday that five of the captives held by Hamas had died, and that the group was still holding 136.
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 detainees, and more aid entered Gaza.
Twenty-five other detainees, mostly Thais, were also freed in separate arrangements.
Seven days of prisoner exchanges had yielded jubilation in the streets of the occupied West Bank as Palestinian detainees, children and women, walked free from Israeli jails.
On Thursday, eight Israeli, some holding dual nationality, were released in the seventh round of exchanges under the truce.
Israel's prison service later said another 30 Palestinian detainees -- 23 children and seven women -- had been freed.
Hamas said it had offered to hand over the bodies of a mother and her two sons -- one of them a baby -- in talks to extend the now-expired truce, but Israel refused.
Israeli officials refused to comment.
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.
The Israeli military said that ground, air and naval forces struck targets in the north and south of the Gaza Strip, including in Khan Yunis and Rafah.
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 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.
The Israeli military published a map of "evacuation zones" in the Gaza Strip that it said would enable residents to "evacuate from specific places for their safety if required".
Residents in various areas of Gaza were sent SMS warnings on Friday.
"Stay away from all military activity of every kind," the warnings said, but their is no safe place in Gaza as Israel strikes everywhere.
Hours after the truce collapsed, Israeli bombardment killed three people in southern Lebanon, one of them a Hezbollah member, according to the resistance group.
Hezbollah meanwhile claimed its first attacks on Israel since the truce ended.