TAL AFAR, Iraq - Insurgents melted into the countryside through a network of tunnels to escape an Iraqi-U.S. force that reported killing about 150 rebels while storming the militant bastion of Tal Afar.

Following the classic guerrilla retreat on Sunday, the city has now been swept clear of extremists for the second time in a year. Iraqi and U.S. military leaders vowed to redouble efforts to crush insurgents operating all along the Syrian frontier and in the Euphrates River valley.

"Tal Afar is just one piece of an overarching operation. We are not going to tolerate a safe haven anywhere in Iraq," said Army Maj. Gen. Rick Lynch, deputy chief of staff for coalition forces in Iraq.

As Baghdad kept a border crossing into Syria closed about 60 miles west of Tal Afar, Defense Minister Sadoun al-Dulaimi issued a warning: "The Syrians have to stop sending destruction to Iraq. We know the terrorists have no other gateway into Iraq but Syria."

While insurgents were retreating from Tal Afar, militants elsewhere killed one U.S. soldier and a British soldier in separate roadside bombings Sunday and assassinated an official in Iraq's Interior Ministry.

A Task Force Liberty soldier was killed and two were wounded during a pre-dawn patrol near Samarra, 60 miles north of the capital. At least 1,897 U.S. personnel have died since the Iraq war started in March 2003.

In the southern city of Basra, a British soldier was killed and three were wounded in an attack on their convoy, the British Ministry of Defense said in London. Britain has reported at least 96 deaths since the war began.

Police said Maj. Gen. Adnan Abdul Rihman, the Interior Ministry's director of police training, was shot in front of his west Baghdad home as he waited for a ride to work.

Tal Afar had been cleared of militants a year ago, but insurgents moved back after U.S. troop numbers in the area were reduced.

U.S. warplanes bombed several suspected militant targets in the city last week, and the long-expected assault to again take Tal Afar was launched early Saturday by 5,000 Iraqi soldiers backed by a 3,500-strong American armored force.

By Sunday night, the joint force reported 156 insurgents killed and 246 captured. It said troops found a big bomb factory, 18 weapons caches and the network of escape tunnels beneath Tal Afar's ancient Sarai neighborhood.

After stiff initial resistance Saturday, insurgents fell back and their stronghold was nearly deserted when the joint force moved in.

"The terrorists had seen it coming (and prepared) tunnel complexes to be used as escape routes," Lynch said.

As troops continued house-to-house searches in Tal Afar, a group claiming to be an offshoot of al-Qaida in Iraq said it would strike U.S. positions and the Iraqi government in Baghdad with "chemical and unconventional weapons ... unless the military operations in Tal Afar stop within 24 hours."

The Mujahedeen of the Victorious Sect posted the threat on an Islamic Web site known as a clearing house for militant messages. The claim could not be authenticated, but it was the second such threat since Friday, when al-Qaida in Iraq said it would use chemical weapons against Baghdad's Green Zone, which houses the Iraqi government, parliament and the U.S. Embassy.

The U.S. military, meanwhile, said it killed a key al-Qaida leader, identified only as Abu Zayd, during a raid on a safe house in Mosul, 45 miles east of Tal Afar.

Most of Tal Afar's residents fled before the fighting.