Four Israeli attack helicopters swooped down on a crowded neighborhood in the Gaza Strip early today and fired missiles at a car carrying a top leader of the Islamic Resistance Movement, killing him and three bodyguards.

In response, the militant movement, known as Hamas, vowed a major escalation in the fight, authorizing its cells in the West Bank and Gaza Strip to strike back "forcefully" and to give "top priority" to attacks on Israel's political leadership, according to a statement by the group. The Hamas leader, Ibrahim Maqadmeh, was a founder of the group and a military strategist.

The Israeli missile strike was the latest attack in a three-week offensive against the group that began after Hamas asserted responsibility for the deaths of four Israeli soldiers, who were killed Feb. 15 when their tank drove over a land mine. Since then, dozens of Israelis and Palestinians have been killed in a series of Israeli and Palestinian attacks. Many, if not most, of the victims on both sides have been civilians.

The latest bloodshed came just hours before the Palestinian leader, Yasser Arafat, officially nominated Mahmoud Abbas, a senior official and independent-minded moderate, to serve as the Palestinians' first prime minister. Abbas, better known as Abu Mazen, was instrumental in negotiating peace accords with Israelis and is considered one of the few Palestinians with the clout and independent base of support to take on Arafat.

The Bush administration, Israel and other international players have said that an alternative Palestinian leadership is crucial to renewing peace talks, ending the cycle of violence and purging the Palestinian Authority of corruption and ties to terrorism. U.S. officials have said they hope an empowered prime minister will dilute Arafat's authority and usher in reform.

Arafat and Abbas attended a meeting today of the PLO's central committee at Arafat's shelled and battered compound in the West Bank city of Ramallah, where the committee will continue through Sunday to debate the powers of the prime ministerial position. The committee has no authority to vote formally on the matter; that will be the responsibility of the Palestinian Legislative Council, which is scheduled to begin deliberations Monday. The council is expected to approve the laws creating the post, and if Abbas accepts the nomination, confirm his appointment.

Abbas, 68, did not comment today on his nomination. He previously has said that he would not decide whether to take the job until he knew what powers came with it.

In a signal of the difficulties Abbas could face in stopping the violence as prime minister, Hamas -- which opposes the Palestinian Authority's efforts to reach peaceful accommodation with Israel -- disparaged the effort, saying, "The [Palestinian Authority] is busy choosing a prime minister in response to American-European pressure, while the Zionist enemy continues its premeditated aggressions against the Palestinian cities, refugee camps and villages."

Witnesses to the strike on Maqadmeh said that he and three bodyguards were killed when the AH-64 Apache helicopters fired five missiles at their car, turning the vehicle into charred rubble, smashing the windows of nearby homes and businesses and scattering body parts across the street.

Saeb Erekat, the Palestinian Authority's minister for local government, told reporters that the missile strike was "the action of a mafia more than a government."

Maqadmeh, who reportedly was imprisoned by Israel for almost a decade, was considered one of the most senior leaders in Hamas, which is headquartered in the Gaza Strip. Although some Hamas officials said today that he had forgone military activities in recent years and was solely a political leader, other Hamas activists said that he maintained a leading hand in political and military operations.

A statement released by Israeli security officials said Maqadmeh "was one of the most extreme of the Hamas leaders over the past two decades" and was "one of the central characters in the planning, approval and executing" of Hamas military operations.

Since the tank attack last month, Israel has launched almost daily strikes against Hamas strongholds in the Gaza Strip, killing dozens of Palestinians. On Friday, Israeli forces seized several square miles of territory in the northern Gaza Strip that Hamas militants have used to launch homemade Qassam rockets at communities just over the border in Israel. The Israeli army has called its operation "open ended."

Hamas responded to Israel's stepped-up offensive by dispatching a suicide bomber who attacked a bus in the port city of Haifa on Wednesday, killing himself, 14 Israelis and an American. One of the more than 40 people injured in the attack died of his wounds today. Hamas also asserted responsibility for killing a couple in their home Friday night in an attack on a Jewish settlement in the West Bank. The two Hamas attackers were killed. At about the same time, two armed Palestinians were killed by Israeli soldiers outside another settlement nearby. In a statement, Hamas said the two settlement attacks were meant to be coordinated in "revenge for the killing of unarmed innocent civilians" in Gaza.

Moore reported from Ramallah.

Rescue workers in Gaza recover the remains of Palestinian militant Ibrahim Maqadmeh and three aides after their vehicle was hit by Israeli missiles.