JENIN, West Bank -- Palestinians ambushed Israeli troops in the cramped quarters of a West Bank refugee camp Tuesday, setting off a suicide bomb trap in a narrow alley and firing on soldiers in a courtyard, the military said. Thirteen Israelis were killed, the biggest blow to the army in its West Bank offensive.

The carefully planned double attack in the Jenin refugee camp also wounded nine Israeli soldiers. A military spokesman said a rapid series of blasts went off in the alley -- one of them detonated by a suicide bomber -- and collapsed part of a building on several soldiers.

Defying U.S. demands that Israel's 12-day-old campaign wrap up without delay, Prime Minister Ariel Sharon vowed "Operation Defensive Shield" will go on until it fulfills its mission -- "the destruction of the infrastructure of the terror groups."

"This is a battle for survival of the Jewish people, for survival of the state of Israel," he said on Israel TV.

There were signs, however, that U.S. efforts were having an effect. Earlier Tuesday, Israel pulled out of Tulkarem and Qalqiliya, two of six Palestinian towns it has occupied; troops remained in Nablus, Bethlehem, Jenin and Ramallah and several villages.

Secretary of State Colin Powell, due to arrive in Israel late Thursday, said he was optimistic his mission could bring a truce and lead to negotiations.

Israel had said it would keep Arafat isolated in the Ramallah offices where he has been confined by troops since the West Bank campaign began. But Israeli officials said Tuesday they would not prevent Powell from meeting the Palestinian leader.

Israel launched its offensive on March 29 to crush militias after a series of Palestinian suicide bombings.

At least 124 Palestinians and 25 Israeli soldiers have been confirmed killed during the incursion, according to Palestinian medics and the Israeli army. The toll was expected to rise.

Aside from the deaths in Jenin camp, an Israeli soldier was killed Tuesday in the city of Nablus, though the military said it may have been by errant Israeli fire.