Israeli helicopters fired missiles at a car in Gaza City Thursday, reportedly killing five people, including an infant, and wounding 25 others. It was the fifth such attack in the past 48 hours.

Initial reports said that the car was carrying Hamas activists. The group claimed responsibility for Wednesday's suicide bombing on a bus in Jerusalem that killed at least 16 people and wounded more than 100 others.

The car with the Hamas activists was driving through the Sheik Radwan neighborhood in Gaza City, on Thursday when it was hit by three missiles. Witnesses said the vehicle was engulfed in flames when it was struck by a fourth rocket.

At the time of the strike, the streets were crowded with mourners who had attended the funeral of 11 people killed in two previous strikes by Israeli attack helicopters.

The operation was launched after Israel's defense minister, Shaul Mofaz, ordered the heads of the nation's security forces to use all means at their disposal to fight Hamas.

On Tuesday, an Israeli helicopter fired missiles at another car in Gaza City, killing two bystanders, in a botched attempt to assassinate Hamas leader Abdel Aziz-Rantissi.

President Bush sharply criticized that operation, saying such actions would not enhance the security of the Jewish state and would make it more difficult for the Palestinian leadership to persuade militant groups to halt their terror attacks.

Despite such criticism, Israeli Prime Minister Ariel Sharon vowed on Thursday to press ahead with operations against Hamas, an Islamic group that is dedicated to Israel's destruction.

Mr. Sharon strongly denounced the Palestinian leadership for failing to act against Hamas. He likened the Palestinian prime minister, Mahmoud Abbas, to a chick that has not grown his feathers yet, and said Israel would continue to fight terror until he does.

On Monday, Mr. Abbas ruled out using force against Hamas and other militant groups, saying this could risk drawing Palestinian society into a civil war.