Twenty-nine people were killed in the first hours after the pause in hostilities ended, according to the Hamas-run health ministry in Gaza.
Israel Defense Forces and international news outlets have reported that rockets were fired from Gaza toward southern Israel.
There have been no reports of casualties from the rockets.
The resumption of hostilities prompted Qatar, a mediator between Israel and Hamas, to call for swift international action against violence in the Gaza Strip.
The Qatari foreign ministry, in a statement, said it stresses that renewed bombing at the end of the pause "complicates mediation efforts and exacerbates the humanitarian catastrophe in the Strip, and... calls on the international community to move quickly to stop the violence."
It added "its condemnation of all forms of targeting civilians, the practice of collective punishment, and attempts to forcibly displace and displace citizens of the besieged Gaza Strip, and its demand for an immediate cease-fire."
Qatar has been engaged in intense negotiations, with support from Egypt and the United States, to prolong a truce in Gaza that had lasted a total of seven days after two extensions.
The agreement was designed to be continued if Hamas could release 10 hostages per day.
Before the truce expired, eight more Israeli hostages were freed on Thursday evening under the terms of the deal.
Qatar's foreign ministry confirmed negotiations between Israel and Hamas "are continuing with the aim of returning to a pause".
The emirate added it was "committed, along with its mediation partners, to continuing the efforts that led to the humanitarian pause and will not hesitate to do everything necessary to return to calm".
During the truce 80 Israeli hostages were freed in exchange for 240 Palestinian prisoners.
Negotiations between Hamas and other nations under a separate framework led to the release of more than 20 other foreign nationals, mostly Thais.
Israel says around 240 hostages were seized when Hamas gunmen stormed across Gaza's militarized border on October 7 and mounted the deadliest attack in the country's history, killing about 1,200 people.
In response, Israel launched a relentless bombing campaign and ground offensive in Hamas-ruled Gaza, which the Hamas government says has killed more than 15,000 people, thousands of them children.
Forum