It was not clear, however, whether the fighting would derail the Cairo talks or the conflict would return to an all-out war.
The Gaza militants fired the first after the truce expired on Friday. They launched 21 rockets into Israel. While most failed, two were intercepted over Ashkelon, a coastal city of Israel. Israeli troops said it responded with strikes across Gaza.
Israel also hit back, targetting a housing project in northern Gaza while another strike hit a farm south of the Gaza City.
Peace mediators are under intense pressure as the two conflicting sides have refused to soften their stands much. While Israel wants Hamas's disarmament, the Hamas seeks Gaza border's opening.