An Israeli airstrike killed a Palestinian in Gaza City shortly before noon on Sunday, putting the death toll of the residents of the coastal enclave to 18 since Friday when a new wave of conflicts erupted.

The dead was a 52-year-old man who guarded an orange orchard where the Israeli missile landed, witnesses said, adding that the missile was targeting a group of militants, who managed to escape the attack.

The militants apparently avenged the strike, as dozens of home- made and Russian Grad rockets were fired from the Gaza Strip in response to the Israeli offensive.

Early Sunday, a 12-year-old Palestinian was killed in an Israeli airstrike in northern Gaza Strip, medical sources said.

Victims of Sunday's airstrikes marked the first two civilian fatalities out of the 18 people who have been killed since Friday.

The violence started when Israel killed the leader of the militant group Popular Resistance Committees (PRC) and his assistant while they were driving in Gaza City.

Israel said it killed the PRC chief to thwart his plans to attack Israel from Gaza's border with Egypt.

Militants responded with rockets toward some Israeli communities near Gaza, and the Israeli drones chased the fighters down, hitting 12 Islamic Jihad militants and four PRC fighters over the past two days.