JERUSALEM, Tuesday, Jan. 21 - The Israeli Army seized control of the Palestinian city of Tulkarm on Monday, and this morning began moving into another West Bank city, Nablus, where five Palestinian militants were killed in exchanges of fire, the army said.

In Tulkarm the army imposed a 24-hour curfew and sent troops door to door in a hunt for militants and weapons. Israeli forces have previously occupied positions in Palestinian-controlled territory, but the move into Tulkarm was the first time in 16 months of conflict that they took over an entire city. Israeli officials said, however, that the soldiers would remain at most for a few days.

Stepping up its actions, Israel sent tanks into a neighborhood of Nablus early today in what it described as a much more limited operation than the one in Tulkarm.

Israeli tanks drove within one- third of a mile of the Nablus city center before dawn. The army said the Palestinians were killed when Israeli troops tried to make arrests in the Al Majeen neighborhood. The army seized a building where an explosives factory was located, military officials said.

Israeli military officials also said today that army units had begun a pullout from Tulkarm.

The raid in Tulkarm came in response to an attack on Thursday in the northern Israeli city of Hadera, not far from Tulkarm. A lone Palestinian gunman walked into a crowded bat mitzvah celebration and opened fire, killing 6 people and wounding 31.

In sporadic shooting on Monday between Israeli soldiers and Palestinian gunmen darting through Tulkarm's twisting streets, at least one Palestinian was killed and nine were wounded. Israeli officials said the soldiers had made several arrests.

Israeli forces occupied several houses, including that of the mayor, Mahmoud al-Jallad. Outside Mr. Jallad's two-story home this afternoon stood a tank, an armored bulldozer and two armored personnel carriers, one of them with an Israeli flag covering its front end.

Israeli officials said the attack in Hadera had been planned and staged from Tulkarm, although the killer was from a village to the east, near Nablus. They said they had invaded Tulkarm, a city of at least 40,000 on the disputed boundary between the West Bank and Israel, because they doubted that Yasir Arafat, the Palestinian leader, would comply with treaty obligations to provide security for Israelis.

``I think we felt that we had to take the initiative, in order not to just sit around and wait for the next terrorist attacks,'' said Zalman Shoval, the former Israeli ambassador to Washington and an adviser to Prime Minister Ariel Sharon. Mr. Shoval left open the possibility that Israel would move into other West Bank cities.

Terje Roed-Larsen, the United Nations' Middle East envoy, warned in a statement on Monday that the Israeli action in Tulkarm ``constituted a dangerous escalation that would lead to more loss of life on both sides.''

Mr. Arafat remained a virtual prisoner in his official compound in Ramallah, where the sound of gunfire from clashes outside could be clearly heard on Monday. A Palestinian intelligence officer was killed in fighting in Ramallah on Monday, Palestinian officials said.

In impassioned remarks to a group of Palestinian intellectuals, Mr. Arafat cited the operations in Tulkarm as he accused the Israelis of having ``crossed all red lines.'' Early Friday morning, after the attack in Hadera, Israel destroyed a large Palestinian government building in Tulkarm with a missile attack by F-16 fighters.

Mr. Arafat said he would continue pursuing the goal of a Palestinian state with its capital in Jerusalem even if it cost him his life.

``May God give me the honor of martyrdom in my steadfastness for Jerusalem,'' he said. The Israeli government has said it has no intention of killing Mr. Arafat, but it will not let him leave Ramallah until he arrests the killers of Israel's tourism minister, who was slain by Palestinian gunmen on Oct. 17.

Mr. Arafat insisted that he remained ready to make ``the peace of the brave,'' but he did not dwell on the cease-fire supposedly in force with the Israelis. ``Our people cannot stand with their eyes closed to these Israeli attempts,'' he said.

A debate has been growing in Israel over whether the government provoked Palestinian violence a week ago by killing Raed al-Karmi, a militant leader in this city. The government has not officially claimed or denied involvement in Mr. Karmi's death, caused by a hidden bomb, but in interviews senior military and political officials have acknowledged that Israel was responsible.

Some officials and critics of Mr. Sharon's government have questioned the timing of the killing, during a period of relative calm after Mr. Arafat called for a halt to violence on Dec. 16.

A senior Israeli military official, speaking on condition of anonymity, said the attack on Hadera had been planned before he was killed - and by Mr. Karmi himself. But he acknowledged that it would ``not necessarily'' have been carried out had Mr. Karmi not been slain.

Mr. Karmi's comrades in Tulkarm insisted that he was abiding by the cease-fire, although he had boasted of killing Israelis in the past.

Israeli officials insisted that in any event, Palestinian militants had violated the cease-fire first, by killing four Israeli soldiers on Jan. 9 at an outpost just outside the Gaza Strip. The Islamic group Hamas claimed responsibility for that attack.

Stores were closed in Tulkarm on Monday, and the streets were eerily empty, except near the Israeli positions in buildings reinforced with sandbags and flying the Israeli flag, or at strategic intersections blocked by armored vehicles. Near such points, Palestinian gunmen gathered to trade shots with the Israelis, and children came to throw stones.

One boy, Khaled Abdul Rahia, was struck in the head at close range by a companion's stone, intended for two tired-looking Israeli soldiers standing by a tank. As he pressed a hand to his bleeding head, Khaled, who gave his age as 13 but looked about 8, expressed no regrets. ``I want to kick them out of here,'' he said.

Soldiers prevented a reporter from entering Mayor Jallad's home on Monday, and they refused to confirm that he was inside. Reached on his cellphone, Mr. Jallad, appointed by Mr. Arafat's Palestinian Authority, said he and his wife and two children had been forced into a downstairs room and were being held there without explanation. He said he was hiding in his bathroom with his telephone to avoid being overheard.

Asked why the soldiers had entered his house, Mr. Jallad, an engineer not known to be linked to militant groups, ventured a guess. ``Because I am the mayor,'' he said. ``They want to send a message to our people and to President Arafat that they can enter the home of the big boss here and catch him.''