The army said it had warned residents to stay indoors.
Palestinian sources said at least 27 people had been killed in an Israeli attack in Gaza. Israel said it was in response to rocket fire from Hamas.
Some 1,460 Palestinians, mostly civilians, have died in the conflict and 63 Israelis, mostly soldiers.
The ceasefire had been brokered by the US and UN to give civilians a reprieve from the violence, and had been seen as an unforeseen breakthrough after days of diplomatic deadlock.
Palestinian and Israeli delegations have arrived in Cairo, Egypt, with the hope of negotiating a longer-term cessation of hostilities, but the fate of the talks now seems unclear.
During the morning life appeared to be returning to some kind of normality in Gaza, with many Palestinians heading towards heavily shelled areas to see if their homes were still intact.
But the Gaza health ministry reported that at least 27 Palestinians were killed and 100 injured in an Israeli attack near the southern town of Rafah after the ceasefire began.
AFP news agency said the artillery barrage was continuing, preventing medics from recovering dead and wounded from the site.
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