Intensive Israeli shelling killed at least 40 people and wounded more than 150 in southern Gaza on Friday, medics said, as a fragile truce was shattered just hours after beginning.
The deaths brought the total toll from 25 days of conflict to at least 1,500 Palestinians, emergency services spokesman Ashraf al-Qudra said.
AFP correspondents said an artillery barrage of the city of Rafah prevented medics from recovering the dead and wounded, after the army said militants had resumed rocket fire against Israel.
Gaza militants fired rockets and mortar rounds over the border as a three-day ceasefire that began at 8:00 am (0500 GMT) was broken.
An AFP correspondent reported fierce fighting in the Rafah area, and Israel's army warned residents to stay in their houses as it "pursued terrorist elements".
Sixty-three Israeli soldiers have been killed in the 25-day offensive, and three civilians inside Israel have died from cross-border rocket or mortar fire.