Israel killed five Palestinian militants in a series of air strikes in the Gaza Strip on Friday after gunmen fired rockets at the Jewish state, Hamas officials and Gaza militants said.
Gaza militants fired about a dozen rockets at Israel, injuring two Israelis, including a man listed in serious condition, Magen David Adom paramedics said.
The death toll in Gaza was the highest in five months, reigniting tensions along a testy frontier and sparking fears of a possible escalation in the violence.
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 three 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, with several militant groups claiming they fired rockets at Israel.
The other three militants killed in three separate air strikes in which Israel said it was trying to stop rocket attacks, were from the Islamic Jihad group.
Gaza militants fired more than a dozen rockets at Israel, a military spokeswoman said, most of them after the strike that killed two militant leaders. Israel's Iron-Dome system intercepted some of the rockets, but one of them injured two civilians, paramedics said.
Israel often launches retaliatory attacks after missiles are fired at its territory from Gaza though such strikes have grown rarer in past weeks.
The army added that Qessi and others 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.
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 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.
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 Jon Boyle)