Israel killed four Palestinian militants in air strikes in the Gaza Strip on Friday after gunmen fired rockets at the Jewish state, Hamas officials and Gaza militants said.
Two militant leaders died in an Israeli military attack on a car in Gaza City launched after two rockets were fired at Israel from the coastal territory, causing no damage or injury, officials said.
Separate air strikes, launched after darkness fell, killed two more militants in Gaza, Hamas Islamist officials in Gaza said.
The Israeli military said one of those killed in the first air strike was Zuheir al-Qessi, who Palestinians said headed the Popular Resistance Committees (PRC), an umbrella group of armed activists from different factions. It named the other militant leader killed in that strike as Mahmoud al-Hanani.
Qessi's death brought a swift call for retaliation from the PRC, reigniting tensions along the testy frontier, with several militant groups claiming they fired rockets at Israel.
Israel often launches retaliatory attacks after missiles are fired at its territory from Gaza though such strikes had grown rarer in past weeks.
The army added that Qessi and other were preparing to attack Israel across the Egyptian border in the coming days, suggesting that the first air strike on the car was ordered to stop the planned assault.
The Israeli military said militants fired eight rockets in all on Friday. Israelis living along the Gaza frontier were urged to stay indoors.
Mahmoud al-Hanani, a local militant leader, was also killed in the first Israeli airstrike, said officials.
Qessi was elected chief of the PRC last August after Israel killed his predecessor in an air strike in the southern Gazan town of Rafah.
Israel on Friday accused Qessi of helping to plan attacks along its border with Egypt that killed seven Israelis last year.
The PRC has denied involvement in that cross-border attack.
Gaza militants vowed to retaliate for Israel's attacks.
All options are open before the fighters to respond to this despicable crime. The assassination of our chief will not end our resistance, Abu Attiya, a spokesman for the PRC group said.
Fawzi Barhoum, spokesman for the Hamas Islamist government in Gaza, held Israel responsible for what he called a grave escalation.
They want us to sink in darkness and in blood, he added.
The last lethal Israeli strike in Gaza was in mid-January when two Palestinians were killed in attacks Israel said targeted a squad planting bombs at the border fence.
(Reporting by Nidal al-Mughrabi; Writing by Allyn Fisher-Ilan; Editing by Andrew Heavens)