Air raid sirens were sounded twice in an hour in southern Israel following the interception of a rocket fired from Gaza, a first since the start of the ceasefire last week.
A bout of gunfire and explosions rocked the northern part of the Gaza Strip just minutes before the truce deadline, according to local media affiliated with Hamas.
The seven-day truce, which began on 24 November, ended with daybreak on Friday, although Qatari and Egyptian mediators said they were still in contact with both sides and trying to keep open negotiations to resume the ceasefire for an eighth day.
The Israeli military said its fighter jets had resumed targeting Hamas militants across the Gaza Strip. At least 32 people were killed and dozens sustained injuries just hours into the renewed offensive, the health ministry in Gaza said.
Palestinian residents said Israel dropped leaflets on Friday morning over parts of southern Gaza, urging people to leave homes east of the town of Khan Younis, which it warned was a “dangerous battle zone”.
The leaflets signalled that Israel is preparing to widen its offensive, which has so far focused largely on the northern part of the Gaza Strip.
Israel had said earlier on Thursday evening that it was open to continuing the ceasefire if Hamas committed to further hostage releases. "We're ready for all possibilities.... Without that, we're going back to the combat," Mark Regev, an adviser to Israeli prime minister Benjamin Netanyahu, told CNN.
After two last-minute extensions to the original four-day ceasefire, the seventh day of the Qatar-brokered truce brought the exchange of eight Israeli hostages and 30 Palestinian prisoners.
Hamas has so far freed more than 100 hostages, including 83 Israelis and 24 foreign nationals, while Israel handed over 240 Palestinians – mostly teenagers accused of throwing stones and firebombs during confrontations with Israeli forces.
Israel says around 125 men were still being held hostage by Hamas and other militant groups in Gaza.
The truce was pivotal for the entry of humanitarian aid into Gaza after much of the besieged region of 2.3 million people was reduced to wasteland due to Israel's incessant bombardment.
More fuel and 56 trucks of humanitarian supplies entered Gaza on Thursday, Israel's defence ministry and the Palestinian Red Crescent Society said. But deliveries of food, water, medical supplies and fuel remain far below what is needed, aid workers say.
Israel has killed over 15,000 Palestinians in Gaza in its bid to destroy Hamas following the militant group's 7 October rampage, according to the Gaza health ministry. Israel says gunmen killed 1,200 people and took 240 hostages during the initial terror attack.
US secretary of state Antony Blinken, during his third visit to Israel since the onset of the war, agreed that the flow of aid into Gaza was not sufficient.
Mr Blinken told the prime minister that Israel cannot repeat in south Gaza the massive civilian casualties and displacement of residents it inflicted in the north.
“We discussed the details of Israel’s ongoing planning and I underscored the imperative for the United States that the massive loss of civilian life and displacement of the scale that we saw in northern Gaza not be repeated in the south,” Mr Blinken told reporters in Tel Aviv.
“And the Israeli government agreed with that approach,” he said. This would include concrete measures to avoid damaging critical infrastructure such as hospitals and water facilities and clearly designating safe zones, he said.