U.S. troops opened fire on a bus carrying civilians Friday in central Afghanistan, killing four passengers after their driver refused to stop, officials said.

A spokesman for NATO's International Security Assistance Force said the bus was heading toward 20 troops on foot patrol on a highway in central Wardak province. The troops first fired warning rounds in the air to stop the vehicle, then shot into the engine block. The bus kept coming, so they opened fire on the vehicle in self-defense.

The provincial governor and area residents confirmed that the troops were U.S. forces.

At least 10 other passengers were wounded, Wardak Gov. Halim Fidai said. An ISAF statement said the wounded were evacuated to military hospitals.

The incident is one of a series that threaten to undermine Afghan support for foreign troops just as the United States prepares to boost its presence in the country. The U.N. said in September that 577 Afghan civilians had been killed this year by U.S., NATO and Afghan troops, a 21 percent jump from 2007. Taliban fighters and other insurgents killed another 800 civilians this year.

The bullet-ridden bus had been moved to the side of the road by the afternoon, and U.S. troops cordoned off the area, according to an AP cameraman on the scene. The windows of the blue bus were shattered, and one side was pocked with holes. People gathered nearby to try to see the wreckage.

Rahmat Ullah, who owns a construction company in Kabul but lives in Wardak, said he could hear the shots at about 10 a.m. from his house. He said he came outside to watch the wounded being transferred into helicopters. The rumor circulating in the crowd was that the driver hadn't been paying attention, Ullah said.

U.S. forces spokesmen declined to comment on the incident, referring all inquiries to NATO's International Security Assistance Force because the troops were operating under its auspices. ISAF spokesmen spoke only on condition of anonymity because they are not allowed to be identified.

The shooting occurred about 40 miles south of Kabul on the main road between the capital and the southern city of Kandahar.

Fidai had said earlier on Friday that the bus was caught in the crossfire of a battle between U.S. forces and insurgents. The NATO force said the patrol did come under fire Friday morning, but that this happened separately from the bus incident.

Wardak and the neighboring province of Logar are two of the areas slated for a new infusion of U.S. forces in January, the first of a potential 20,000 forces that U.S. commanders have requested.

U.S. military officials have said that insurgents are increasing attacks in the area in order to create a sense of insecurity at the gates of Kabul.

American troops have been in Afghanistan since 2001, when the U.S.-led coalition ousted the hard-line Taliban regime.