More than 100 Israeli troops entered the West Bank city of Tulkarm before dawn Wednesday, searching for members of the militant Palestinian group behind a suicide bomb attack the previous day in Israel.

Scores of jeeps and armored personnel carriers took up positions around the city, which Israel turned over to Palestinian control four months ago under a truce that has been badly shaken by recent violence. As the most intensive military operation of the five-month-old cease-fire began, Israeli troops killed an armed man and critically wounded a second in clashes on the edge of the city, in the northwestern part of the West Bank. Palestinian officials identified the men as police officers.

Around midnight, Israeli special forces entered the West Bank city of Nablus and killed Mohammed Saswat Assi, 24, a senior commander of the al-Aqsa Martyrs Brigades, an armed wing of the ruling Fatah faction. Another man was arrested. Israeli military officials said the soldiers surrounded a house and saw one man trying to escape. After calling out to him to stop, soldiers shot and killed him, the military said. Military officials identified the man arrested as an Islamic Jihad operative, but Nablus residents said he was a member of al-Aqsa.

Israeli officials described the Tulkarm operation as the start of a sustained effort against Islamic Jihad, the group that asserted responsibility for the suicide bombing Tuesday outside a shopping mall in the city of Netanya, less than 10 miles west of Tulkarm on the Mediterranean coast. The death toll from the bombing climbed to four Wednesday when an Israeli woman died of her wounds.

"We handed over Tulkarm with the expectation that the Palestinian Authority would attack and arrest terrorists there," said Raanan Gissin, a spokesman for Israeli Prime Minister Ariel Sharon. "They did nothing. So today, we did it."

The Israeli military presence in Tulkarm marks a setback for the halting process of reconciliation that began in February with a cease-fire agreement between Sharon and the Palestinian leader, Mahmoud Abbas. The move comes as Sharon is preparing to evacuate all 21 settlements in Gaza and four in the northern West Bank, a process known as disengagement that is under threat from radical Palestinian groups and Jewish extremists.

Hoping to calm rising domestic tensions over the plan, Sharon on Wednesday declared the Gaza settlements a closed military zone. The decision, which effectively seals off the area to non-residents, came five days before thousands of activists planned to march to the Gaza settlements and remain there to resist the evacuation.

Gissin said the decision was meant to reduce the "potential for friction," but settler leaders reacted sharply to the closure. The order also applies to the settlements in the northern West Bank, but Israeli officials said it would not be enforced there until later, given the difficulty of closing the area because of its more open terrain.

"This is the first time in modern history that a Jewish leader is imposing a siege and blockade against Jews," said Shaul Goldstein, the deputy chairman of the Yesha Council, the umbrella organization of Jewish settlements in the West Bank and Gaza.

Sharon, once the inspiration behind the movement to settle lands occupied by Israel in the 1967 Middle East war, favors evacuating the Gaza Strip for security and demographic reasons. He has said it has become too costly to protect Gaza's 8,500 Jewish settlers from the strip's 1.3 million Palestinians.

But many Palestinian leaders suspect that Sharon intends to use his withdrawal from Gaza to strengthen Israel's hold on the West Bank, a region with far more religious and strategic significance to Israel. A debate over the country's long-term intentions has divided the Palestinian leadership over whether to support Sharon's disengagement project -- designed without consulting the Palestinians -- or oppose it.

Abbas, who supports Israel's withdrawal from Gaza, has warned that Islamic Jihad might try to derail the process. Among the smallest and most extreme of the Palestinian factions, Islamic Jihad agreed to abide by the February truce, which called for Israel to hand over five Palestinian cities reoccupied during the uprising, or intifada, that began in September 2000.

But only Tulkarm and Jericho had been handed back to Palestinian security services because, Israeli officials say, Abbas has not moved against the armed groups that threaten Israel. Fighters from Islamic Jihad's military wing, which has criticized Israel for failing to fulfill its obligations under the truce, have attacked settlers in the West Bank and soldiers in Gaza since the agreement took effect. In that time, Israeli forces have arrested more than 300 Islamic Jihad activists and killed one of its leaders near Jenin in the northern West Bank.

In a statement Wednesday, Khalid Batsh, an Islamic Jihad leader in Gaza, said the Netanya attack was "a natural response to the ongoing Zionist violations against the Palestinian people."

Saeb Erekat, the chief Palestinian negotiator with Israel, called the operation in Tulkarm "very unfortunate," asserting that "whoever planned and carried out this act in Netanya had exactly this in mind. They wanted to sabotage progress toward the peace process, any smooth transition in Gaza, and the future turnover of other areas to Palestinian control."

At 3 a.m. Wednesday, the Israeli military notified Palestinian officials in Tulkarm that soldiers were entering the city. Infantry troops, border policemen and undercover units are in Tulkarm, Palestinian officials said, and some Israeli soldiers have taken up positions on the tops of buildings. A curfew has been imposed, and Israeli forces have arrested a half-dozen men. Two Israeli soldiers have been injured in the operation, Israeli officials said.

Izz a-Din Sharif, Tulkarm's governor, said the operation in Tulkarm amounted to unfair punishment for the Tuesday mall attack. He said the Netanya bomber, identified as Ahmed Abu Khalil, 18, came from the village of Atil, five miles east of Tulkarm. Sharif said the Israeli military remained in charge of the village after Tulkarm was handed over to the Palestinians.

"What we see today in Tulkarm is a retaliatory act done for the Israeli public opinion," Sharif said in a telephone interview. "That area was under their military rule. So how can they hold us responsible?"

Israeli police gather at the gateway into Jewish settlements in the Gaza Strip to stop non-residents from entering. Israeli soldiers take up position during a military operation in Tulkarm, where they searched for members of the militant Palestinian group Islamic Jihad.