A Hamas policeman inspects the remains of a vehicle after it was targeted by an Israeli air strike that killed the head of the Popular Resistance Committees militant group in Gaza City
Twelve Palestinians, most of them fighters, were killed and at least 20 wounded in a series of Israeli air strikes on the Gaza Strip, Palestinian medics said early Saturday.
The Israeli raids came as Palestinians fired dozens of rockets and mortar rounds into southern Israel starting Friday morning, injuring four people, one seriously, Israeli military sources said.
One Israeli strike, on a car travelling in the Tel El-Hawa neighbourhood west of Gaza City, killed the head of the militant Popular Resistance Committees, Zohair al-Qaisi, and fellow member Mahmud Hanani, the group said.
The PRC threatened to retaliate for Qaisi's death, and around 40 rockets and shells were subsequently fired on southern Israel, reports AFP.
The Al Quds Brigades, the military arm of Islamic Jihad, said later that strikes had killed 10 of its members.
The PRC and the Al-Aqsa Martyrs Brigades, an armed offshoot of Palestinian president Mahmud Abbas's Fatah movement, issued statements claiming to have fired rockets into Israel on Friday.
The official Palestinian WAFA news agency quoted a statement by the Fatah-dominated Palestinian Authority condemning the Israeli retaliation, saying it had created a "negative environment" that would "escalate the circle of violence in the region."
The Israeli military said Qaisi "was among the leaders who planned, funded and directed" a deadly cross-border attack into southern Israel from Egypt's Sinai last August.
In that incident, gunmen carried out a coordinated series of shooting ambushes on buses and cars on Route 12, which runs along the Egyptian border 20 kilometres (12 miles) north of the Red Sea resort of Eilat.
The shootings took place over several hours, leaving eight dead and more than 25 wounded.
The military statement said Qaisi was also involved in a 2008 attack on a terminal for pumping fuel from Israel into the Gaza Strip, in which two Israeli civilians were killed.
The statement added that both the dead men were "responsible for planning a combined terror attack that was to take place via Sinai in the coming days."
It said that other strikes were aimed at men about to fire rockets into Israel.
Hamas, which rules the Gaza Strip, has maintained a tacit truce with Israel, but other armed Palestinian groups regularly fire rockets and mortars across the border, which can spark air strikes in response.