A Palestinian suicide bomber pulled a car packed with explosives alongside a moving bus and blew it up Wednesday in a huge fireball that killed 17 passengers — 13 of them Israeli soldiers. The military warned that Palestinians were shifting tactics to kill more Israelis each time they strike.

In an apparent retaliatory move hours later, Israeli armored vehicles entered the West Bank town of Ramallah early Thursday and surrounded the office of Palestinian leader Yasser Arafat amid sounds of gunfire, witnesses said.

The Islamic Jihad group claimed responsibility for the bombing and said it was timed to mark the 35th anniversary of the 1967 Mideast War, when Israel captured the West Bank and Gaza Strip. The terror group said the bomber, Hamza Samudi, was from the West Bank town of Jenin, and Israel responded by sending tanks there on Wednesday.

The suicide bomber was 16, a relative said, making him one of the youngest bombers to strike Israel.

In Israel, there were growing calls for the army to reoccupy the West Bank and to move against Islamic Jihad headquarters in the Gaza Strip.

The Palestinian Authority condemned the bombing in a statement that for the first time underscored it had no advance knowledge of the attack. Palestinian officials said they were under orders to arrest members of the group.

In warning Israelis of new tactics from terrorist foes, the army spokesman said Palestinians had planned to release lethal cyanide gas as part of a suicide bombing in March that killed 29 Israelis. Brig. Gen. Ron Kitrey said the gas was not used because of technical difficulties.

He said plans to employ the poison were part of continuing efforts by Palestinians to carry out what he termed a "mega attack."

Last month, Palestinians detonated a bomb under a tank truck as it was refueling at Israel's largest fuel depot. The explosion burned the truck's cabin, but did not spread to other vehicles or the huge fuel containers. Israeli officials said they recently thwarted plans to park an explosives-filled truck under twin high-rise buildings in Tel Aviv.

Israeli Prime Minister Ariel Sharon delayed his scheduled departure for the United States, canceling weekend meetings in New York. He plans to leave Saturday night for talks in Washington Monday with President Bush, the prime minister's office said.

In a harsh statement, the White House condemned the attack and said it raised questions anew about Arafat's relevance to the Middle East peace process.

"In the president's eyes, Yasser Arafat has never played a role of someone who can be trusted or effective," presidential press secretary Ari Fleischer said.

Wednesday's blast went off at about 7:20 a.m. near the town of Megiddo — the Hebrew word for Armageddon, scene of the final battle between good and evil in the Book of Revelations.

"In the blink of an eye, I saw a car passing and then there was an explosion," said Sharon Levinger, a soldier who was in the front of the bus. Bus driver Mickey Harel, who survived three previous attacks, said he felt his bus "leap and then turn over."

Passengers were hurled onto the pavement as the bus tumbled; others, including a man and woman embracing in their final moments, died trapped in the burning vehicle. Body parts and personal belongings scattered well away from the bus as rescue workers in white overalls collected human remains.

The soldiers killed in the attack were in their late teens and early 20s. Thirty-eight other people were hurt, 10 seriously. The attacker was also killed.

The Islamic Jihad has a history of striking at soldiers inside Israel — viewing them as legitimate military targets; that notion is rejected in Israel, where most 18-year-olds are automatically drafted into the military.

It was the deadliest attack since Israel ended its six-week offensive in the West Bank last month. The mission underscored the ability of suicide bombers to strike despite the arrest and killing of many of their number during and after the Israeli incursion.

The Islamic Jihad leader in Gaza, Abdullah Shami, called the attack "part of our resistance," and group's overall leader, Ramadan Shalah, said in Damascus, Syria, that it was meant to coincide with the 35th anniversary of the war in which Israel captured the West Bank, Gaza and east Jerusalem, claimed by the Palestinians.

It also came a day after CIA chief George Tenet met Arafat to press him to carry out reforms that would make his security apparatus more effective in preventing terror attacks. Arafat presented a reform plan, but Israeli media reports said Tenet considered it insufficient.

A Palestinian official said Tenet warned Arafat that if the Palestinian leader did not prevent terror attacks, he would stand alone in facing Israeli reprisals — an apparent threat that the United States would give Sharon a freer hand in retaliating.

Sharon convened his security Cabinet after the attack. Sharon spokesman Raanan Gissin said that "there will be no sanctuary and no safe haven for any terrorist."

Several ministers have demanded that Israel expel Arafat, and Israeli President Moshe Katsav demanded the international community cut off all ties with him.

The bombing and the expected Israeli reprisal were likely to complicate U.S. efforts to formulate a Mideast initiative, including a proposed regional conference with the aim of restarting peace talks. A Bush administration official said the United States hoped to organize the meeting in Turkey to reopen talks that were frozen in January 2001, days before Sharon was elected.

Egyptian President Hosni Mubarak was to be in Washington Friday for a meeting with Bush.

Wednesday's attack marked the first time that a suicide bombing was carried out with a car in motion, alongside a moving target. In 1994, a member of the Islamic group Hamas detonated a car bomb seconds after pulling alongside a parked bus in Israel.

Reuven Paz, an Israeli counterterrorism expert, said the use of a car allowed terrorists to use more explosives than in the typical suicide bombing where attackers wear explosives belts.

Palestinian security officials said two dozen armored vehicles drove into Jenin from three directions. Two attack helicopters fired machine guns toward one neighborhood, witnesses said. There were no immediate reports of injuries.

Israeli forces have surrounded most Palestinian cities and towns in the West Bank, and make almost daily raids to go after suspected terrorists. Checkpoints dot the West Bank and Gaza, severely restricting movement for most Palestinians.

Still, Palestinian bombers have been able to carry out dozens of bombing attacks inside Israel since the current Mideast fighting erupted in September 2000. This week, the government announced plans to build a fence along part of the border in an effort to keep them out.