JERUSALEM - Twelve people were shot and killed last night in the West Bank city of Hebron when Palestinian gunmen ambushed a group of Jewish worshipers as they walked home from a prayer service and then traded fire with Israeli soldiers guarding them.

Doctors at two Jerusalem hospitals were treating 15 other victims, some of them critically injured, who were flown in on Army helicopters. It was not immediately clear exactly how many civilians were among the casualties. Three gunmen were later found dead.

The intense, 90-minute gunfight in downtown Hebron near the Tomb of the Patriarchs, a shrine revered by Jews and Muslims, and in the rugged hills above made it difficult to evacuate the wounded.

The radical Islamic Jihad militant group claimed responsibility for the attack.

Senior army commanders and Israel's defense minister spoke by telephone early today with Prime Minister Ariel Sharon at his ranch in the Negev desert to formulate a military response, which officials said would most likely include a reoccupation of the Palestinian part of divided Hebron.

It was the second deadly attack by Palestinians in a week. On Monday, a gunman killed five Israelis in a kibbutz in northern Israel, prompting the army to return to the West Bank city of Nablus, where it plans a lengthy reoccupation.

Foreign Ministry officials called last night's attack the "Sabbath massacre" because the weekly day of rest begins Friday at sundown. The faithful had just left the tomb and were walking on what is popularly called Worshiper's Lane to their homes in Kiryat Arba, adjacent to Hebron.

Details of the battle were difficult to ascertain immediately. Many of the injured were too badly hurt to talk and others refrained from giving interviews because it would violate the Sabbath. But the scant information available provided a picture of a harrowing and frantic scene.

"There was gunfire from left and right, from every possible angle, they were shooting at us from above," a man in the group identified only as Arik told Israeli Radio. "I didn't know where to go. I fell flat on the ground. It was a slaughter."

Army commanders described the attack as well-planned, with shooting coming from at least two directions and aimed not only at the worshipers but at an army post.

As gunfire and grenades rained down from the hillside, rescue workers circled armored ambulances and military vehicles to shield the worshipers and evacuate the wounded and dead. Authorities said the gunmen shot at rescuers as they tried to treat the wounded and that casualties included border police and paramedics.

Meanwhile, soldiers returned fire and shot up flares to illuminate the night sky.

Shortly before midnight, Israeli soldiers fired on a house from which they saw shots coming and killed two gunmen. Later they found a third body in the area. The army said two Kalashnikov rifles and grenades were found near the bodies in the house.

The Islamic Jihad, which is backed and funded by Iran, told Al-Jazeera television that two of its members had carried out the attack in retribution for last month's killing by the Israeli army of the head of its military wing, Iyad Sawalha.

The organization claimed that one of the soldiers killed was a high-ranking army commander, possibly a lieutenant colonel, who heads the Hebron brigade. Hezbollah Television in Lebanon broadcast a name, but the Israeli army had no comment.

About 450 Jewish settlers live surrounded by 130,000 Palestinians in Hebron in a volatile mix of ideology and extremism. The Jewish residents are among the most militant - some still belong to outlawed underground militias - and clashes occur almost daily, despite the round-the-clock deployment of hundreds of police and soldiers.

Both Jews and Muslims claim the city on religious grounds. The Tomb of the Patriarchs is sacred to both religions as the burial site of Abraham, Jacob and Isaac. The army allows each side to alternate prayer days; part of the tomb is a mosque.

Jews were driven out of Hebron during Arab riots in 1929, when dozen were killed. Settlers began to move back into the Jewish Quarter after Israel captured the West Bank in the 1967 Arab-Israeli War. Kiryat Arba was built in 1972 and is home to thousands of Israelis.

In 1994, Baruch Goldstein, a New York-born settler, killed 29 Muslim worshipers inside the mosque at the tomb and was then beaten to death by survivors.

Three years later, as part of an interim peace deal, sections of Hebron and surrounding hilltop villages were turned over to the Palestinian Authority and the city was divided into sectors for Israelis and Palestinians.

Palestinians have routinely shot at Israelis in Hebron and on worshipers, many times with fatal results. In March 2001, 10-month-old Shalhevet Pass was hit in the head and killed by a Palestinian sniper. But settlers also are instigators, and they routinely clash with the army and police assigned to protect them.

In August, Hebron settlers returning from a funeral for one of four Israelis killed in a roadside ambush went on a rampage in a Palestinian marketplace, setting fire to stores and damaging buildings. A 14-year-old Palestinian girl was shot dead as she stood in her doorway. Israeli police were preparing for similar confrontations today.

Hebron residents also have repeatedly complained that the Israeli Army has failed to prevent attacks such as the one that occurred last night. The army had reoccupied all of Hebron for months as part of its sweep through the West Bank after a series of suicide bombings in July.

But soldiers were pulled out of most of Palestinian Hebron on Oct. 25 when former Defense Minister Benjamin Ben-Eliezer struck a deal with Palestinian security officials in which the army would withdraw in exchange for a promise of quiet.

Earlier this week, Jewish leaders in Hebron sent a scathing letter to the army listing 11 attacks that have occurred since its withdrawal, including several shootings and a stabbing of a police officer.

"There can be no doubt that the Palestinians have violated the conditions demanded of them upon the Israeli retreat," the letter said. "For these reasons, we demand that the [army] be returned to all of Hebron immediately, prior to another tragedy."

Though the army had left the city, it kept two posts on the hilltop in Abu Sneineh to prevent gunmen from shooting at the settlers below. The gunmen in last night's attack apparently eluded the soldiers and managed to stay hidden throughout the protracted battle.

Elsewhere in the West Bank, the Israeli Army continued to operate in several Palestinian cities in its bid to eradicate militant groups. Troops fatally shot teen-agers in Nablus and Tulkarm, saying each had thrown a firebomb at a tank. The army also said it had arrested a youth in Nablus who was on his way to carry out a suicide bombing.

In Ramallah, soldiers invaded a banquet hall and arrested 17 members of the militant group Hamas as they ate a meal to break the daily Ramadan fast.

This comes as officials of Palestinian leader Yasser Arafat's Fatah party tried during meetings in Cairo to persuade Hamas to end attacks inside Israel.

A formal agreement was not reached, but any cease-fire would not have included Islamic Jihad, which carried out the Hebron shooting. And even Fatah's armed faction, the Aqsa Martyr's Brigades, has vowed to continue attacks even against orders from its political wing.