A four-wheel-drive vehicle laden with about 175 pounds of explosives rammed into the rear of a commuter bus on a busy highway in northern Israel today, turning the bus into a fireball that burned some passengers alive and set off the ammunition of soldiers riding inside. At least 14 people were killed, along with the bomber, and 50 were injured in the rush-hour attack, Israeli police reported.

The ferocity of the blast flattened a half-dozen nearby cars and sent the vehicle carrying the bomb flying over the bus. The only pieces of the vehicle that remained intact -- the engine block, one axle and a wheel -- landed 80 yards down the road. A shredded leg -- believed to be that of a Palestinian man police said was probably the driver -- protruded from the debris.

"I saw half bodies. I saw people between life and death. I saw people who had been thrown out the windows," said Asi Dayan, 30, who was driving near the bus and said his car was set ablaze by the explosion. "I saw bodies on the road, I saw people with blood running."

Islamic Jihad, a radical Palestinian movement, asserted responsibility for the attack in a letter faxed to a news agency in Beirut and said the bombing was in "retaliation for the series of massacres committed by the criminal enemy against our people." It listed several recent Israeli military operations that have killed civilians in the Gaza Strip and West Bank. The group added that the attack was timed to commemorate the day seven years ago when Israeli operatives assassinated its founder, Fathi Shiqaqi.

Today's explosion was the first bombing attack inside Israel in 11 days. It threatened to undermine this week's visit by U.S. envoy William J. Burns, an assistant secretary of state. His trip comes at a time when the Bush administration, while preparing for possible war against Iraq, is pressuring the Israeli government to ease military operations against Palestinians.

"The president condemns the most recent attack in Israel," said White House spokesman Ari Fleischer. "It's another reminder of how it's so important for peace to be pursued and for terror to be stopped."

Several senior Israeli officials immediately suggested the military should continue full-scale operations despite recent promises to relax curfews. Public Security Minister Uzi Landau, who visited the scene of today's carnage, said curfews and closures should now be implemented "more forcefully."

Zalman Shoval, a senior adviser to Israeli Prime Minister Ariel Sharon, said the Palestinian leader, Yasser Arafat, is responsible for the bombing because he is "not making it clear to the different Palestinian groups that violence and terror have to stop."

Arafat, at his compound in the West Bank city of Ramallah, told reporters: "You know that the Palestinian leadership is against such attacks that target civilians, Israelis or Palestinians."

Today's blast occurred at about 4:30 p.m. during rush hour at the Karkur Junction, an intersection with a shopping mall on the outskirts of Pardes Hanna-Karkur, near Hadera and about 30 miles north of Tel Aviv. It was the fourth time a car bomb has exploded near a bus in this area, near the Green Line separating Israel and the West Bank. Seventeen people were killed when a car slammed into a bus just outside the nearby town of Afula on June 5.

Car bombs have become a trademark of Islamic Jihad, which has claimed responsibility for a large percentage of the more than 20 car bombings that have been used in attacks against Israeli buses, marketplaces, city centers and a Tel Aviv disco since the Palestinian uprising against Israeli occupation in Gaza and the West Bank began in September 2000.

Col. Daniel Kuffler, assistant commander for the Israeli police's northern district, said 10 attacks have occurred in this vicinity in the last two years. This region, just west of the West Bank city of Jenin, has been so terrorized by the attacks that the Egged bus company, whose No. 841 bus was the target of today's explosion, had organized security patrols of the bus stops.

An Egged security vehicle, which stood near the bus stop where today's bombing occurred, was damaged in the blast, company officials said.

No. 841 began its trip near Israel's northern border with Lebanon just before 2 p.m. It was making its way down a route through Jewish and Arab communities on its way to Tel Aviv when it pulled into Karkur Junction, according to company officials and police. Witnesses said a vehicle cut across traffic and pulled up behind the bus as it stopped for passengers.

"Suddenly there was an explosion," said Dayan, who was returning from a trip to the Arab town of Umm el-Fahm, where he was selling pharmaceuticals. "I was shaken by the concussion. My car caught fire and I jumped out. I saw flames from the bus and from other cars."

Passengers scrambled out of the bus. But before those at the rear could escape, the bus exploded in flames, burning some passengers alive before rescue workers could get near. The bus was left a charred skeleton.

More than four hours after the blast, rescue workers were still attempting to pull bodies from what remained of the bus, while others collected body parts blown 100 yards in all directions. Metal shards from cars, pieces of clothing and shattered glass littered the highway, which was burned black by the explosion and fire.

The attack was the first inside Israel since a failed bombing attempt on a bus outside Tel Aviv on Oct. 10, when a suicide bomber slipped and fell while trying to clamber aboard a commuter bus. The bomber stumbled away from the bus and detonated his explosives, killing an elderly woman who was walking nearby.

Moore reported from Jerusalem. Researcher Samuel Sockol contributed to this report.

Israelis carry a victim away from the bombed bus at Karkur Junction. The Islamic Jihad group claimed responsibility for the attack.