Israeli troops discovered an underground room being used by Palestinian militants as a hideout in the West Bank city of Nablus on Saturday and pounded it with grenades and gunfire, killing six senior leaders from three Palestinian groups, Israeli army officials and Palestinian security sources said.

It was one of the deadliest attacks on a group of senior guerrilla leaders since the Palestinian uprising began in September 2000. The dead included the West Bank leader of the al-Aqsa Martyrs Brigades, Nayef Abu Sharekh, who was in his forties, and the leader of the Islamic Jihad in the northern West Bank, Fadi Bahty, 26, according to Palestinian security and hospital officials.

Jaffar Masri, a senior member of the Islamic Resistance Movement, known as Hamas, was also killed in the attack, Palestinian officials said, as were three other senior leaders of al-Aqsa Martyrs Brigades, an armed faction linked to Palestinian leader Yasser Arafat's Fatah political movement.

Israeli officials said Abu Sharekh, Bahty and Masri were the masterminds of numerous suicide bombings that killed dozens of Israelis.

Late Saturday, the body of a seventh, unidentified man was discovered in the rubble, an Israeli army spokesman said.

Two other Palestinian men, including another senior al-Aqsa Martyrs Brigades leader, were killed by Israeli troops Saturday in Nablus, a city known as a center of Palestinian militancy about 28 miles north of Jerusalem. The killings came on the third day of an Israeli army operation in Nablus and raised the toll to 11 Palestinians killed and 24 injured, Palestinian hospital officials said.

Israeli troops invaded the old city in the center of Nablus on Thursday, where they imposed a curfew and have been engaged in rooting out Palestinian guerrillas and weapons workshops. The operation was launched after Israeli police apprehended a would-be suicide bomber from Nablus in a neighborhood in north Jerusalem on Tuesday. Later that day, Israeli soldiers at a West Bank checkpoint discovered a backpack loaded with explosives in the trunk of a taxi headed from Nablus to Jerusalem.

The new crackdown is one of the most intensive in Nablus in a year. Residents contacted by telephone said local mosques were blaring messages from loudspeakers urging citizens to attack Israeli troops who were manning road blocks across the city and mounting house-to-house searches in the old city. Israeli soldiers earlier in the week distributed leaflets demanding that guerrillas turn themselves in and asking Nablus residents to inform on them. The leaflets warned that if the militants did not give themselves up, Israeli soldiers would use all means to capture them, residents said.

Nablus's old city is a warren of narrow alleys and tunnels that Palestinian fighters have used to evade capture and frustrate many Israeli search operations, and the Israeli strike on the hideout was a bit of a fluke, according to Palestinian security sources.

Israeli soldiers on patrol in the old city at about 3 p.m. stumbled upon a senior member of the al-Aqsa Martyrs Brigades, Nidal Wawi, about 35, as he was emerging from a tunnel. Equally surprised, Wawi and the soldiers both opened fire, and Wawi was killed, Palestinian security officials said. Israeli soldiers then mounted an intense search, they said, and a few hours later discovered the hidden room, which they destroyed with grenades, killing six Palestinians.

An Israeli army spokeswoman offered a slightly different version of events. She said Israeli troops spotted two armed men in a house, and a firefight erupted, during which one Palestinian was killed and the second escaped down a tunnel hidden in the house. Soldiers threw smoke bombs into the tunnel, the spokeswoman said, flushing out a militant from another tunnel entrance in a nearby house.

The soldiers discovered a room two floors below ground and called for the others inside to come out, but they refused, so the soldiers threw grenades into the room and opened fire, killing at least six senior leaders. Late Saturday, Israeli soldiers discovered a seventh body, she said.

Earlier in the day, Ehab Asleem, 17, was shot and killed in the courtyard of his home, Palestinian officials said. An Israeli army spokeswoman said Asleem was armed, and troops opened fire before he could get off a shot. Asleem's father and brother also were shot and seriously injured in the incident, Palestinian hospital officials said.