Israeli aircraft have attacked four targets in the Gaza Strip, lightly wounding three people, Palestinian security sources and witnesses said.
They said the targets were a mortar launch installation near Gaza City, a group of militants in the north of the strip, a disused Hamas intelligence headquarters near Gaza City and a rocket-launching site near Shati refugee camp.
The casualties were from the attack on the intelligence building, witnesses said.
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The Israeli military could confirm only one strike, on what a spokeswoman called "a terror target" in the northern Gaza Strip.
She said the raid was in response to Palestinian rocket attacks on Israel.
Earlier, Defence Minister Ehud Barak warned that the Jewish state had no choice but to respond to the rockets and mortar shells being rained down on southern Israel by Gaza militants.
"We have to respond," Barak said at a joint news conference with visiting US Defence Secretary Robert Gates shortly after two Grad rockets slammed into the southern port city of Ashdod.
"Israel will not tolerate these terrorist attacks and we will not allow terror to rise once again," Barak said.
Since the weekend, dozens of rockets have hit southern Israel, the vast majority fired by Islamic Jihad's military wing the Al-Quds Brigade, who on Wednesday fired two Grads at Beersheva 40 kilometres (25 miles) away.
As tensions spiralled in and around the coastal enclave, Gaza's Hamas rulers had on Wednesday vowed to rein in recalcitrant militant groups and restore calm.
But by Thursday afternoon, at least 11 rockets and mortar rounds had landed in southern Israel, the army said, two of them ploughing into Ashdod, Israel's fifth largest city with a population of more than 200,000.
Israel's response has so far been muted, with the military launching air strikes and raids which have caused only limited casualties.
But Barak said Israel would respond to the assault, and insisted it would choose both the timing and the nature of its response to the rising tide of rockets, while taking care not to become "victims of our own resolution."
"We keep the right -- how, when and in what kind of amount of firepower or munitions -- to respond," he said.
Thursday's rocket fire on Ashdod came a day after the Al-Quds Brigades vowed to fire more at cities deep inside Israel as it entered "a new phase" of resistance.
