Palestinian leader Mahmoud Abbas (search) pledged Saturday to track down and punish militants responsible for a suicide bombing that killed four Israelis and wounded 50 people at a Tel Aviv nightclub. Palestinian security officials pointed to the Lebanese guerrilla group Hezbollah as the apparent mastermind.

Israel and the United States said Abbas has to act quickly and forcefully — despite his long-standing reluctance to confront militants — if he wants to rescue a fragile Mideast truce.

The bomber was identified as Abdullah Badran, 21, a university student who according to relatives had no history of militant activity.

Palestinian security officials said they were investigating whether Badran was recruited by local militants from the Al Aqsa Martyrs' Brigades (search), a violent group with ties to Abbas' ruling Fatah movement, at the behest of Hezbollah. The Lebanese group has stepped up efforts in recent weeks to disrupt the truce declared Feb. 8.

The bomber detonated an exceptionally powerful bomb — radio reports said it was about 66 pounds — around 11:30 p.m. among Israelis waiting outside the seaside nightclub Stage.

"Suddenly there was this huge explosion and we just ran," said Merav Ayush, 20, who was on her way to the club when the explosion occurred. "I saw a boy and a girl sitting on the ground. At the entrance to the club there were about 15 people just lying on the floor," Ayush said from her hospital bed.

Police said the guards outside the club spotted the bomber and didn't let him in. The explosion dispersed thousands of metal shards that had been packed into the bomb, cutting down people standing nearby. The explosion ripped off the front of the nightclub, shattering windows of nearby restaurants and blackening cars.

The bodies of a young woman and the bomber lay under white plastic bags in a pool of blood and shattered glass covered the sidewalk. Three of the wounded died on their way to hospital, paramedics said.

Israeli Defense Minister Shaul Mofaz (search) was to meet with army chiefs after the end of the Jewish Sabbath at sundown Saturday, but it appeared there would be no immediate Israeli retaliation. Gideon Ezra, the police minister, said the Palestinians must "do much more to prevent such attacks" but that contacts with the Palestinian Authority would continue.

Abbas pledged to track down those responsible for the attack, accusing them of trying to derail the peace process. "The Palestinian Authority will not stand silent in the face of this act of sabotage," Abbas said in a statement after holding an emergency meeting early Saturday with his security chiefs. "We will follow and track down those responsible and they will be punished accordingly."

There was no official claim of responsibility, but Palestinian officials and militant leaders said Hezbollah had orchestrated the attack. Asked whether Hezbollah was involved, Palestinian Prime Minister Ahmed Qureia said Saturday: "I don't know. I'm not sure."

Hezbollah declined comment. "As far as we are concerned, there is no need to respond to such lies," a Hezbollah official said in Beirut.

Early Saturday, Israeli troops entered the village of Deir al Ghusun and arrested two of Badran's brothers and four other village residents, including the local mosque preacher, Palestinian security officials said. The Badran family said it was informed by the army that Abdullah was the Tel Aviv bomber.

Badran, a student of education at a local university, had no known ties with militant groups, said his brother, Ibrahim.

The army confirmed the arrest raid, but gave no further details.

In Washington, Secretary of State Condoleezza Rice condemned the attack "in the strongest possible terms" and said it is essential that Palestinian leaders take "immediate, credible steps" to find those responsible. Rice took note of the Palestinian condemnation of the attack. "We now must see actions that send a clear message that terror will not be tolerated," she said.

Hezbollah has emerged as the biggest threat to the fragile Israeli-Palestinian truce, offering West Bank gunmen thousands of dollars to attack Israelis. The group, which is backed by Iran and Syria, has hundreds of West Bank gunmen on its payroll, according to Palestinian security officials.

Hamas, Islamic Jihad and the Al Aqsa Martyrs' Brigades, the largest Palestinian militant groups, all denied involvement, saying they were respecting the informal truce.

"If it turns out that indeed an individual from the brigades carried out his attack, everyone must know that they acted alone, and that the Aqsa Brigades does not identify with him and does not recognize him as a member," said Abu Mahmoud, the group's official spokesman.

However, a senior Al Aqsa commander said one of its members had been recruited by Hezbollah to carry out the attack. The commander spoke on condition of anonymity.

A senior Palestinian security official, speaking on condition of anonymity, also said the bomber was hired by Hezbollah. Palestinian officials said they had been tracking communications between Hezbollah and Al Aqsa militants in the northern West Bank in recent days.

The Tel Aviv promenade has been hit before by Palestinian militants, including explosions in 2001 outside the Dolphinarium disco and Mike's Place, a popular pub.

Since this month's Mideast summit, the two sides have stepped up security cooperation, and Israel has pledged to hand over security responsibilities in most of the West Bank to Palestinian forces.

The two largest and most powerful Palestinian militant organizations, Hamas and Islamic Jihad, have so far refused to join a cease-fire officially, but have pledged to maintain the fragile calm.

The radical Palestinian factions are expected to hold talks with Egyptian officials next week on the cease-fire with Israel, a senior Hamas official said on condition of anonymity.

While Israel has welcomed Abbas' efforts, it wants the Palestinian leader to begin to take steps to dismantle the militant groups — a step he is reluctant to take.

"What we need now is action, and not words," said Gideon Meir, a senior Foreign Ministry official.

It was the first suicide bombing since the cease-fire declaration, as well as the first such attack since the death of longtime Palestinian leader Yasser Arafat on Nov. 11. The last suicide bombing killed three people in a Tel Aviv market on Nov. 1.