Israeli aircraft bombed Hamas targets in the Gaza Strip the Islamist group said, after Prime Minister Ehud Olmert vowed a "disproportionate" response to mortar fire that injured three Israelis.
The aircraft carried out three strikes but there were no casualties.
One attack was on a security headquarters in a village in central Gaza that residents said had been vacated after Israel telephoned warnings to Palestinians to leave buildings that housed any weapons.
Two strikes were on suspected sites of tunnels along the border with Egypt, Hamas said. The Israeli military had no immediate comment.
The Israelis wounded by mortar fire included two soldiers and the first Israeli civilian hurt since an 18 January truce ended Israel's 22-day offensive in the coastal enclave.
About a dozen rockets and mortar bombs fired from Gaza struck Israel on Sunday, most causing no casualties.
A wing of al-Aqsa Martyrs Brigades, a group belonging to Palestinian President Mahmoud Abbas's Fatah faction, said it fired some of the rockets, but not all were claimed.
Israeli leaders have taken a hard line against rocket fire.
"The government's position was from the outset that if there is shooting at the residents of the south, there will be a harsh Israeli response that will be disproportionate," Olmert, who is not a candidate, said at the weekly cabinet meeting.
A spokesman for the Hamas government in the Gaza Strip condemned what he described as Mr Olmert's "aggressive statement".
But the spokesman, Taher al-Nono, also urged all Palestinian factions to "respect the national consensus" on the ceasefire the Islamist group declared two weeks ago after Israel announced it was halting the Gaza offensive.
Israel was criticised internationally for the deaths, according to the Hamas-run Health Ministry in Gaza, of more than 1,300 Palestinians, including at least 700 civilians.
Critics said Israel had responded disproportionately, in its air and ground offensive in heavily populated areas, to cross-border rocket attacks over eight years that killed 18 people.
Ten Israeli soldiers and three civilians were killed in the Gaza campaign.
Israel said Hamas militants bore responsibility for civilian deaths in Gaza by operating inside its towns and refugee camps.
Since the truce, in addition to Sunday's injuries, an Israeli soldier was killed and three others were wounded when a bomb exploded next to their patrol. Israeli air strikes since 18 January have killed three Palestinians and wounded 10.
