Palestinian gunmen have killed a pregnant Israeli woman and her four children in an attack near a Jewish settlement in the Gaza Strip. Observers say the killings could influence the outcome of Sunday's referendum of the ruling Likud Party on a plan to have Israel withdraw from the territory and parts of the West Bank.

The gunmen launched their deadly ambush as the Israeli woman was driving with her children on a road that leads to the Gaza Strip settlement of Gush Katif. Everyone in the car reportedly died instantly in the hail of automatic gunfire.

The attackers set off an explosive charge at a nearby Israeli army post, but troops were able to give chase. They intercepted and killed two Palestinians in a gun battle.

Israel called the ambush a massacre and accused Palestinian leader Yasser Arafat and Palestinian armed groups of trying to turn Likud Party voters against Prime Minister Ariel Sharon's plan to pull out of Gaza.

Israeli spokesman Avi Pazner says the attack was the slaughter of "innocent children."

Two Palestinian groups claimed responsibility for the killings - the Islamic Jihad, listed by the State Department as a terrorist organization, and the Popular Resistance Committees, an umbrella group responsible for other attacks in the Gaza Strip.

The ambush occurred just as voting began among nearly 200,000 Likud members on Mr. Sharon's proposals for Israel to unilaterally withdraw all Jewish settlements and troops from the Gaza Strip and parts of the West Bank.

Observers note that even before Sunday's ambush, there were many in the Likud who firmly opposed the Sharon plan.

Among them is Uzi Landau, a Likud member of Mr. Sharon's cabinet, who says the attack only serves to strengthen the case against Israel giving away more territory to the Palestinians.

"The risks are enormous. There are no gains. Only terrorists are gaining from it," he said.

Mr. Sharon, once hailed as one of the biggest supporters of the Jewish settler movement, says he no longer accepts that Jews have a right to settle in Gaza.

He says the evacuation is necessary for the sake of Israel's overall security.

As Likud voters cast their ballots at more than 400 polling booths across the country, he warned that a rejection of his plan in the referendum could place Israel's future at risk.