JERUSALEM Ñ Palestinian insurgency groups intend to launch an offensive against Israel in wake of the Likud Party rejection of a plan to unilaterally withdraw from the Gaza Strip and much of the West Bank.

Israeli officials said Palestinian insurgency groups have been cooperating to launch a wave of attacks against Israeli civilian targets.

They said that over the last 48 hours Israel's intelligence services received 50 alerts of Palestinian attacks.

The planned Palestinian offensive would use the Gaza Strip as well as such West Bank cities as Jenin, Nablus and Hebron as launching pads for attacks. Officials said the Palestinian Authority, including chairman Yasser Arafat, has been cooperating with the planned insurgency campaign.

On Sunday, five Israelis -- a pregnant social worker and her four children -- were killed and three others were injured in an attack near the Israeli bloc of communities in the central Gaza Strip. The family was driving toward Israel along a heavily-protected road when they were ambushed and lost control of the vehicle. The Palestinian insurgents then approached the disabled vehicle and killed all the occupants.

Israeli troops and combat vehicles rushed to the scene and encountered the insurgents. At least two Palestinians were killed in the Israeli military response.

Witnesses said the family was driving to Israel as part of an effort to lobby members of the ruling Likud Party to oppose Prime Minister Ariel Sharon's plan for a unilateral withdrawal from the Gaza Strip and large parts of the West Bank. Sharon's withdrawal plan was defeated by about 60 percent of the Likud membership in what was termed a stinging blow to the prime minister and an embarrassment to U.S. President George Bush, who endorsed the plan. Sharon stressed that he would not resign in wake of the referendum.

"I know that much of the Israeli public supports my plan," Sharon said. "I know that they feel, as I do, disappointment with the results of the referendum. We have difficult days before us where difficult decisions need to be made. One thing is clear to me. Israel did not elect me to sit and do nothing for four years. I was elected to find a way to bring quiet, security and peace to this nation."

Islamic Jihad claimed responsibility for the strike. Jihad said the attack comprised a joint operation with the ruling Fatah movement to avenge the Israeli assassination of Hamas leaders Ahmed Yassin and Abdul Aziz Rantisi in March and April 2004.

"We know that this attack was inter-organizational as most of the recent attacks," Chief of Staff Lt. Gen. Moshe Ya'alon, who visited the scene, said.

Earlier, Israeli Defense Minister Shaul Mofaz said Hamas has been desperate to launch a major attack against an Israeli target in wake of the assassination of Yassin and Rantisi. Mofaz told the Cabinet that Hamas's failure to stage a major strike was undermining the credibility of the leading Islamic insurgency group in the West Bank and Gaza Strip.

Israel's military has increased the pace of its operations over the last 24 hours in an effort to prevent Palestinian attacks. On late Sunday, Israeli AH-64A Apache helicopters fired missiles toward a Hamas radio station in Gaza City. The station had broadcast interviews with Hamas leaders and reported the movements of Israeli troops.

In the northern West Bank city of Nablus, an Israeli helicopter fired missiles that killed four Fatah insurgents, two of them described as senior operatives. It was the first Israeli helicopter attack in the West Bank in nearly two years. Overnight Monday, Israeli troops arrested 31 suspected Palestinian insurgents in the West Bank.