Israeli troops entered this West Bank (search) town Thursday and witnesses said they arrested a local leader of Islamic Jihad, pushing forward with an offensive against the Palestinian militant group following a homicide bombing that killed five Israelis.

About 35 jeeps, backed by Apache helicopters, entered Jenin in the afternoon, and troops surrounded the home of Abdel Khalim Izzadin (search). After a brief standoff, Izzadin and three other men surrendered to troops, witnesses said.

The army confirmed it had carried out an operation in Jenin, and said the troops were withdrawing.

Israel (search) launched an offensive against Islamic Jihad earlier Thursday, a day after the homicide bombing at an outdoor market in central Israel. Islamic Jihad claimed responsibility for the attack.

Prime Minister Ariel Sharon had promised a "broad and nonstop" response to the bombing.

The offensive will include airstrikes and artillery attacks in Gaza and arrest raids in the northern West Bank, where Wednesday's bomber came from, a military official said on condition of anonymity under military regulations. As a last resort, Israel could re-enter Gaza, which it evacuated last month. Israeli media reported that troops would also retake Palestinian towns, and conduct house-to-house searches.

The threatened Israeli response to the bombing in the central town of Hadera ratcheted up pressure on Palestinian leader Mahmoud Abbas to confront militant groups. Abbas has refused to crack down on armed groups such as Islamic Jihad, fearing civil war.

Sharon said the military operation was necessary because of Abbas' refusal to take action and said it would be impossible to resume peace talks until the Palestinians rein in the militants.

"Unfortunately the Palestinian Authority has not taken any serious action to battle terrorism," Sharon said before meeting the visiting Russian foreign minister, Sergey Lavrov. "We will not accept under any circumstances a continuation of terrorism. Therefore our activities will be broad and nonstop until they halt terrorism."

"The state of Israel would very much have liked to move peace efforts forward," he added. "To my regret, as long as terror continues we shall not be able to move forward as we would have wished."

The four airstrikes in the Gaza Strip targeted open fields used by militants to fire rockets, the army said.

Islamic Jihad said Wednesday's bombing was to avenge the killing of one of its West Bank leaders earlier in the week.

The homicide bombing embarrassed Abbas, who hours before the attack demanded that the militant groups stop violating a cease-fire declared in February.

The small Islamic Jihad group signed on to the truce last spring but has repeatedly flouted the cease-fire by claiming it has the right to retaliate for any perceived Israeli violations. It has carried out four suicide bombings inside Israel since the truce.

The much larger Hamas militant group, which plans to run in January parliamentary elections, has largely scaled back its attacks since the truce declaration. In contrast, Islamic Jihad is not participating in the vote and has much less to lose by continuing to attack Israel.

Israeli officials accused archenemies Iran and Syria of assisting the attackers, noting that Islamic Jihad is funded by Tehran and is headquartered in Damascus.

"This infrastructure is murderous and we will try to deal with it and silence it," Amos Gilad, a senior Defense Ministry official, told Israel Radio.

The attack came hours after Iranian President Mahmoud Ahmadinejad praised suicide bombings and said Israel should be "wiped off the map."

Sharon called for Iran to be tossed out of the United Nations for the president's comments, which drew wide international condemnation.

Sharon made the call during a meeting with Russian Foreign Minister Sergey Lavrov.

"The prime minister said that a state which calls for the destruction of another people cannot be a member of the United Nations," according to a statement released by his office.

Sharon and Defense Minister Shaul Mofaz approved the latest offensive in a series of overnight telephone calls, Foreign Ministry spokesman Mark Regev said.

"Islamic Jihad has declared war on every Israeli civilian and of course we're 100 percent entitled to take the appropriate action to defend our civilians," Regev said.

"Ultimately, Israel still hopes the Palestinian Authority will follow through on their own commitments to disarm these groups and that will make the necessity for Israeli action superfluous."

Israeli media compared the operation to Operation Defensive Shield of April 2002, launched in response to a bombing in a hotel that killed 29 Israelis on the eve of the Jewish holiday of Passover.

During that offensive, Israel retook control of Palestinian towns and cities, killing and arresting dozens of militants in house-to-house arrest sweeps. In more than two weeks of fighting, 208 Palestinians and 25 Israeli soldiers were killed.

Capt. Yael Hartmann, an army spokeswoman, said security services were bracing for more attacks planned by Islamic Jihad. As part of its response, Israel sealed off its crossings with Gaza and declared a complete closure on the West Bank.

The measure prevents laborers from entering Israel, keeps Palestinians from visiting relatives in Israeli prisons, and blocked Palestinian goods from exports. The closure came just a day after similar restrictions, in effect for the monthlong period of Jewish holidays, had been lifted.

In Qabatiyeh, the suicide bomber's hometown, the army arrested the attacker's father overnight along with four other Islamic Jihad militants, Palestinian security officials said. The army confirmed the arrests, but declined to say whether the bomber's father had been detained.

The bomber, Hassan Abu Zeid, 20, attacked a food stand in Hadera's open-air marketplace, killing five Israelis and wounding 30. Three people remained in critical condition Thursday.