International troops opened fire on a bus carrying Afghan civilians early Monday, killing four people and setting off anti-American protests in a southern city that is a hotbed of the Taliban insurgency.

Another 18 people were wounded in the incident in Kandahar province’s Zhari district, provincial government spokesman Zelmai Ayubi said. He said international forces took 12 of the wounded to a military hospital. NATO said it was investigating the shooting.

Within hours, scores of Afghans had blocked the main highway out of Kandahar city with burning tires, chanting “Death to America,” and calling for the downfall of Afghan President Hamid Karzai, himself a Kandahar native.

“The Americans are constantly killing our civilians and the government is not demanding an explanation,” said resident Mohammad Razaq. “We demand justice from the Karzai government and the punishment of those soldiers responsible.”

Afghan civilians said the convoy that fired on the bus was American, although NATO and Afghan authorities declined to identify them by nationality. Mr. Razaq and others claimed the bus was fired on merely because it had pulled up close behind the convoy.

Such popular anger threatens to undercut local support seen as crucial for the success of a long-anticipated allied operation to push the Taliban out of Kandahar, the largest city in southern Afghanistan from which the hardline Islamic movement emerged as a political and military force in the 1990s.

Kandahar spokesman Ayubi said the provincial government strongly condemned the shooting. NATO spokesman Jeff Loftin said the alliance had dispatched a team to the scene to investigate, but didn’t say whether its troops were responsible for the shooting.

Mst. Sgt. Loftin said the local command in Kandahar had no further information on what had happened.

The top NATO commander in Afghanistan, U.S. Gen. Stanley McChrystal, has issued strict orders to his troops to try to reduce civilian casualties. But these still occur regularly, unleashing raw emotions that highlight a growing impatience with coalition forces’ inability to secure the nation after more than eight years of war.

With troop levels rising amid heightened violence, at least 2,412 Afghan civilians were killed in fighting last year, an increase of 14 percent from 2008, according to the United Nations. The U.N. attributed 67 percent of those deaths to insurgents who use ambushes, assassinations and roadside bombs to spread terror, undermine development and punish Afghans seen as cooperating with foreign forces and the Karzai government.

NATO earlier this month confirmed that international troops were responsible for the deaths of five people, including three women, killed on February 12 in Gardez, south of Kabul. An Afghan government report on the incident claims U.S. special forces had mistaken their targets and later sought to cover up the killings by digging bullets out of bodies, according to investigators who requested anonymity because they were not authorized to speak with the media.

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