* Israel says air strikes aimed to thwart cross-border attack
* Gaza gunmen fire 30 rockets at Israel, three injured
* Violence is worst in months (Recasts with latest toll)
By Nidal al-Mughrabi
GAZA, March 9 (Reuters) - Israeli air strikes killed 10 militants in the Gaza Strip on Friday and Palestinian gunmen fired more than two dozen rockets at the Jewish state, officials said, in the worst violent flareup along the restive frontier in months.
The violence escalated after Israel blew up a car in Gaza City, killing two militant leaders, shortly after two rockets were fired at Israel from the coastal Hamas-ruled territory, officials said. The rockets caused no damage or injury.
Separate air strikes, launched after darkness fell, killed eight more militants in Gaza, Hamas Islamist officials in Gaza said. Israel confirmed launching three air strikes in all.
Hamas, an Iranian-backed Islamist group that refuses to recognise Israel, seized control of Gaza from Palestinian Prime Minister Mahmoud Abbas's Fatah movement in a bloody 2007 coup, two years after Israel pulled its forces from the territory it had captured in a 1967 war.
The Israeli military said its first air strike killed 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 eight other militants killed in ensuing air strikes, some of which Israel said it launched to thwart rocket attacks, were from the Islamic Jihad group.
Gaza militants fired 30 rockets at Israel, injuring three civilians, one of them seriously, and five other rockets were intercepted by Israel's "Iron-Dome" anti-missile system, a military spokeswoman said.
WORST VIOLENCE IN MONTHS
The death toll in Gaza was the worst since an incident in which five were killed late in October, reigniting tensions along a testy frontier and sparking fears of an escalation in violence in a region also bordering on Egypt.
Some analysts saw the flareup as an attempt by Israel to prevent Gaza militants from using Egypt's Sinai desert as a staging area for attacks. Unlike the fortified Israel-Gaza frontier, Egypt's border with Israel is more penetrable.
The army said Qessi, the most senior militant leader targeted on Friday, had been planning an attack on Israel across the Egyptian border in the "coming days," suggesting that the strike had been ordered to stop the planned assault.
Israel also 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.
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.
After Qessi was killed, 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)