TULKAREM, West Bank - Israeli troops reoccupied the West Bank city of Tulkarem early this morning, killing a Palestinian policeman in a firefight and arresting five Islamic Jihad activists after the militant group killed four Israelis in a suicide bombing.

The bombing Tuesday killed four people outside a shopping mall in the coastal city of Netanya and rattled a 5-month-old truce.

However, Israel, the Palestinian Authority and most Palestinian militant groups have an interest in minimizing violence ahead of Israel's withdrawal from the Gaza Strip and four West Bank settlements next month.

Israel today closed the settlements marked for evacuation to all nonresidents to prevent protesters from reaching the area ahead of the pullout, set to begin in mid-August.

In response to the bombing, Israeli Prime Minister Ariel Sharon ordered security forces to step up operations against Islamic Jihad, which has claimed responsibility for a series of attacks in recent weeks, and to "hit the leadership" of the group. "We will not stop until they (Islamic Jihad) stop the terrorist murders," he said.

The army also sealed off the West Bank and Gaza Strip, barring all Palestinians from entering Israel, a routine measure after attacks. Troops entered the West Bank town of Tulkarem in search of the bombing's masterminds and blew up a storage shed they said housed an explosives laboratory.

Israel handed Tulkarem to Palestinian control four months ago as part of what was to have been a gradual Israeli withdrawal from West Bank cities. Israeli Cabinet ministers said today that the process was frozen after the attack.

Israel also demanded that Palestinian leader Mahmoud Abbas - long reluctant to act against militants - finally crush Islamic Jihad, a small militant group that has repeatedly violated the cease-fire.

The bomber was identified as 18-year-old Sami Abu Khalil. On Tuesday evening, Abu Khalil walked toward the Sharon Mall in Netanya, and detonated 22 pounds of explosives he had strapped to his body. He killed two 16-year-old girls and a 31-year-old woman at a crossing near the mall. A 50-year-old woman died in the hospital today.