A military spokeswoman told AFP that two rockets hit southern Israel during the late afternoon and early evening -- several hours before a 24-hour truce was to expire -- and two more were intercepted by missile defences.
Israel ordered its negotiators back from ceasefire talks in Cairo and the military said warplanes hit Gaza. They hit at least 10 targets, according to army radio.
The fighting shattered nine days of relative quiet in the skies over Gaza and cast a dark shadow over Egyptian-mediated efforts to hammer out a longer-term truce.
The Palestinian delegation presented their demands for a truce to Egyptian mediators and were awaiting Israel's response, said the official, Azzam al-Ahmed.
More From This Section
"There has been no progress," he said of Tuesday's talks. "Matters have become more complicated."
The renewal of Israeli strikes spread panic among Gaza residents.
An AFP reporter saw hundreds of Palestinians streaming out of Shejaiya, an eastern area of Gaza City which has been devastated by more than a month of fighting between Israel and the militant Islamist Hamas movement.
More poured out of the Zeitun and Shaaf areas, alarmed by a series of explosions and heading to shelter in UN schools, local witnesses said.
There was no immediate claim of responsibility for firing the rockets, two of which hit near the city of Beersheva, which is home to around 200,000 Israelis.
An Israeli official said the negotiating team had been ordered back from Cairo where Egypt has been pushing for a decisive end to the Gaza bloodshed, which has killed more than 2,000 Palestinians and 67 on the Israeli side.
"The Cairo process was based on the premise of a total ceasefire," another official told AFP. "If Hamas fires rockets, the Cairo process has no basis."
Israel has vowed not to negotiate under fire, and Prime Minister Benjamin Netanyahu has warned there would be "a very strong response" to any resumption of rocket attacks.
Hamas dismissed his remarks as having "no weight".