KABUL, Afghanistan — U.S. and Afghan security forces fired on protesters in the Afghan capital after a riot erupted today because of a deadly traffic accident involving U.S. troops, leaving at least four people dead, police and eyewitnesses said.

Sher Shah Usafi, a Kabul police chief, said at least three people were killed and 16 wounded in the crash, while U.S. forces killed one person and wounded two as they fired on dozens of rioters shouting "Down with America!" and throwing stones at them while they fled the area.

A commander with the city's traffic police who was at the scene said he also saw U.S. forces firing on protesters. He spoke on condition of anonymity because he was not authorized to speak to the media.

A U.S. military spokeswoman, Lt. Tamara D. Lawrence, confirmed U.S. troops were involved in the accident but said she had no reports of the soldiers opening fire.

An AP reporter at the site said he saw Afghan police firing into a crowd, but the U.S. troops had already left the area.

The incident comes after criticism by Afghan officials and human rights activists who said a U.S. airstrike that killed at least 16 civilians earlier this month — possibly as many as 34 — undermines President Hamid Karzai and boosts support for the resurgent Taliban.

"It's damaging for the dignity of the government," said Noorulaq Homi, a lawmaker from Kandahar province. "The people distance themselves from the government and move toward the Taliban. It is a positive message for the enemy."

Civilians were killed May 21 in an airstrike on Azizi village in Kandahar province. Although the strike killed up to 80 militants by U.S. military estimates, the accompanying civilian casualties can be used as a recruiting tool by Taliban-led rebels.

They also place Karzai, Afghanistan's U.S.-backed president, in a political fix. He remains reliant on the U.S.-led coalition to protect his government but can't ignore the public anger stirred by military mistakes. The airstrike came just five weeks after the killings of seven civilians by coalition military in eastern Kunar province that also drew Karzai's ire.

The risk of civilian casualties appears to have heightened as security forces and militants, who often hide among their ethnic Pashtun brethren, have gone head-to-head in some of the deadliest combat since the hard-line Taliban regime was ousted by U.S.-led forces in late 2001.

Militants operating in the Taliban's former southern heartland have stepped up attacks, testing the will of thousands of NATO soldiers preparing to take over security operations from the U.S.-led coalition there this summer.