The U.S. military launched a major pre-dawn assault Sunday to wrest the northern city of Tall Afar from insurgents but encountered almost no resistance, leaving uncertain the whereabouts of fighters who have battled U.S.-led forces for months.

About 2,000 men -- two battalions from the 3rd Brigade, 2nd Infantry Division, and a battalion from the Iraqi National Guard -- pushed into Tall Afar at 3:15 a.m. to confront what U.S. military officials had expected would be about 200 insurgents who had taken over the local government.

Instead, the U.S. forces, backed by F-16 fighter jets, encountered only brief fire from small arms, U.S. military officials said.

An expected counterattack at dawn, when U.S.-led troops would no longer have the advantage of night-vision equipment, also did not materialize. "We thought there would be more, the indications were that there would be more, but there wasn't," said Brig. Gen. Carter Ham, the commander of U.S. forces involved in the operation. "There's some good news in there, and there's probably some bad news."

Ham said U.S. commanders concluded that some of the insurgents had probably fled in anticipation of the attack. Others, he said, probably gave up after being pounded by three U.S. airstrikes after the operation began on Thursday. It continued into Friday morning before a pause in the fighting.

"And then, thirdly, there is some indication that perhaps we killed more than we think we did [in] the first couple of operations," Ham said in an interview at Camp Freedom, a U.S. military base set up in a palace that once belonged to Saddam Hussein's sons, Uday and Qusay.

Officials with Task Force Olympia, which includes the 2nd Infantry Division, have estimated that 67 insurgents were killed in the initial operation to retake Tall Afar.

Radie Alkhalil, the health minister for Nineveh province, which includes Tall Afar, said the director of the city's hospital told him that 42 Iraqis had been killed since Thursday, including one who died Sunday. Alkhalil said he did not know how many were civilians.

U.S. officials consider Tall Afar, a predominantly Shiite Muslim city of about 250,000 people between Mosul and the Syrian border, a strategic transit point for foreign insurgents entering Iraq to battle U.S.-led forces.

In recent weeks, according to U.S. military officials, hundreds of Sunni Muslim extremists, Baath Party holdovers and foreign insurgents combined to rout the local police force and render the government ineffective. U.S. officials vowed to put down the insurgency and reinstall a legitimate government.

But U.S. forces were unable to accomplish that Sunday, essentially leaving the U.S. troops as the authority in the city. Ham said he hoped to have a new mayor installed within two days. He attributed the delay to "friction and failures of coordination." The main problem, he said, was the inability of U.S. and Iraqi authorities to replace the dissolved Tall Afar police force with 600 officers from the province.

U.S. officials say they believe it is crucial to reestablish authority over insurgent-controlled areas before nationwide elections scheduled for January.

"Having us stay there is exactly the wrong thing," Ham said. "First of all, we don't have enough forces to stay in the city. But it also sends a message to those that oppose us. It lets them say, 'See, we told you, they really are occupiers. They've taken over a city.' "