An Israeli air strike on the Gaza Strip killed the head of the Popular Resistance Committees militant group on Friday, the movement said.
The PRC said General Secretary Zohair al-Qaisi was one of two Palestinians killed in the strike, along with fellow-member Mahmud Hanani, as Palestinians and Israelis traded cross-border fire during the day.
Medical officials told AFP a third man was seriously wounded in the attack on a car travelling in the Tel El-Hawa neighborhood, west of Gaza City.
The PRC vowed to avenge the killings, which came in response to Friday morning mortar fire from Gaza into Israel. Hours after the air strike two more projectiles slammed into the Jewish state, police said, adding that it was not immediately clear if they were rockets or mortar rounds.
Nobody was reported injured in the fire from Gaza.
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 some 20 kilometers 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."
Earlier on Friday, Palestinian militants in Gaza fired two projectiles into southern Israel without causing any casualties or damage, the military said.
A spokeswoman initially said they were rockets, but army radio later referred to them as mortar shells.
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.
The relatively small Popular Resistance Committees is one of the most active, and pledged to avenge its men's deaths.
"We are not committed to the truce; we will respond very strongly to this (Israeli) crime," Abu Ataya, a spokesman for the PRC's military wing, the Al-Nasser Salahadin Brigades, told AFP.
Before Friday's air strike, Israeli army radio quoted what it called "senior military sources" as saying the army "does not intend to allow the firing to continue."
On Wednesday, Israeli troops entered northern Gaza, briefly sealing the Erez border crossing between the Palestinian territory and Israel, Hamas officials said.
The Israeli military said it had entered the area "to stop terrorist activities."
Israeli troops regularly make brief incursions into Gaza, despite formally withdrawing from the territory in 2005.
They usually do so to raze homes or trees near the border that they claim are used by militants trying to infiltrate Israel, fire rockets or plant explosives along the border area.