Three Israeli soldiers were killed and at least 18 people were injured today when a Palestinian suicide bomber's explosives were set off just outside Ariel, one of the largest Jewish settlements in the West Bank. The explosive belt the bomber was wearing apparently was detonated by bullets fired at him by Israelis who confronted him, witnesses said.

Both the al-Aqsa Martyrs Brigades and the Islamic Resistance Movement, known as Hamas, two Palestinian groups that have asserted responsibility for many suicide bombings in the past two years, initially claimed the bomber as a member. They also said the bomber came from the West Bank city of Nablus, about 30 miles north of Jerusalem.

About three hours after the bombing, undercover Israeli soldiers entered Nablus to arrest a Palestinian militant and a firefight erupted in which at least two and possibly three armed Palestinian men were killed, Israeli military officials and Nablus residents said. Residents said the shooting appeared to be a targeted assassination of a militant from the radical Islamic Jihad organization.

The Ariel bombing and Nablus shootings did not appear to be related, Palestinians and Israeli officials said.

Meanwhile, apparently after hearing about the bomb blast 12 miles away at Ariel, a small band of Jewish settlers from the West Bank settlement of Itamar -- where several Jewish residents have been killed in Palestinian attacks this year -- assaulted a group of Palestinian olive pickers and foreign peace activists in the village of Yanun, pelting them with stones and beating them with their guns. Six peace activists -- four foreigners and two Israelis -- were injured.

The suicide bombing today was the second such attack in six days. On Oct. 21, two Palestinian youths from Jenin, a city in the far north of the West Bank, rammed a sport-utility vehicle loaded with 175 pounds of explosives into the back of a crowded bus at an intersection in northern Israel, killing 14 Israelis and themselves.

Israel responded to that attack early Friday by reimposing an around-the-clock curfew in Jenin and sending about 1,000 soldiers into the city and its surroundings. About 90 snipers are nested in various strategic spots in the city, and are shooting any armed Palestinians they see, a senior Israeli commander said Saturday. At least six armed Palestinian men were shot and wounded by Israeli marksmen in the first two days of the operation, he said.

In Jenin today, a Palestinian man, Fuad Abu Rali, 21, was shot and killed when he was spotted leaving his house during the curfew while armed, an Israeli military spokesman said. Palestinian sources in the town and Rali's family said he was unarmed and on the roof of his home when he was shot.

Today's violence followed a recent trip to the region by U.S. Assistant Secretary of State William J. Burns, who was discussing a new plan to lower tensions by bringing Palestinians and Israelis back to the negotiating table and aiming toward the creation of a Palestinian state with temporary borders by next year. Israeli and Palestinian officials greeted the plan with barely disguised derision.

The Palestinian Authority, the governing organization headed by Yasser Arafat, did not specifically condemn today's suicide blast, but issued a statement denouncing the killings of all Palestinians and Israelis.

Saeb Erekat, a top official of the authority, said the statement reflected "the same line the president issues every time" there is a suicide attack, and that nothing should be inferred by its lack of specificity.

"The suicide bombing in Ariel is a clear indication that instead of pursuing a path to peace, Palestinian terrorists have drawn up a blueprint for terror," said David Baker, an official in the office of Israeli Prime Minister Ariel Sharon.

Today's bomb blast occurred at a highway gas station and commercial complex several hundred yards from the main entrance to Ariel, a Jewish settlement of 18,000, located 20 miles north of Jerusalem and about 10 miles inside the West Bank.

The commercial complex, which also includes a supermarket, restaurant and hotel, is a central meeting area and bus stop for Israeli reserve soldiers posted at Jewish settlements around Nablus who generally travel to their posts on Sundays.

A Palestinian suicide bomber blew himself up at the complex's hotel on March 7, injuring about 15 people, including Tova Gilboa, the wife of the man who owns the complex.

Today, at about 11:30 a.m., her husband, Menachem Gilboa, apparently noticed a suspicious-looking man and concluded that he might be a suicide bomber, witnesses said. Aided by an employee from the gas station, Gilboa tackled him.

There are confused and conflicting reports about the melee that ensued and the events that triggered the explosion.

As they struggled on the ground, "I heard Menachem yelling, 'Terrorist! Terrorist! Shoot at the terrorist!' " said Yekhiel Hazan, 43, a resident of Ariel.

Eliezer Biton, a bus driver being treated at Belinson Hospital for shrapnel wounds to his left elbow, said he noticed the fight and started running toward the scene, loading a pistol as he went. As he approached, Biton said, Gilboa and two soldiers had the bomber on the ground, and another man was standing over them, shooting a pistol point-blank at the head of the bomber, whom he described as a large man gripping the end of a white wire in his hand.

"I was a meter away," Biton said. "I heard three shots and then I heard a fourth one and I think there was a big explosion."

Some witnesses said a second man also was firing at the bomber during the incident, and that the explosion occurred when the bomber was shot in the stomach.

Police said it was unclear how many people fired shots. They said they were also investigating whether the bomber detonated himself during the scuffle. "We heard that the terrorist was indeed hit by a bullet, but we're not sure yet if that caused the explosion," a police official said.

Sources said that the three Israeli men killed in the blast, whose identities had not been released last night, were all army soldiers and included two majors.

While it is not unusual for radical Palestinian groups to issue competing claims of responsibility for suicide bombings against Jewish settlers, the claims for today's bombing came with a curious twist as both Al-Aqsa and Hamas initially identified the supposed bomber by different names. Al-Aqsa later said the man was not one of its members. Hamas said he was member Mohammed Bustami, from Nablus.

Researcher Samuel Sockol in Ariel contributed to this report.

An Israeli policeman guards a gas station at the entrance to the West Bank settlement of Ariel, where a suicide bomber's explosives went off. An ultra-Orthodox Jew removes human remains from a gun at the site of a bombing near the West Bank settlement of Ariel.