U.S. troops say Afghan security guards attacked them

KABUL, Afghanistan (Reuters) - U.S. forces in Afghanistan said on Monday they had launched an air strike that killed a number of private Afghan security guards only after coming under fire from that position.

Afghan provincial authorities said on Sunday they were investigating reports that 20 private security guards had been killed in a U.S.-led coalition air strike southwest of Kabul.

A provincial government source said U.S.-led forces called in the strike to fend off an attack by Taliban insurgents on several posts of the local security company that guards a road construction project in the Giro district of Ghazni.

But a spokesman for U.S. forces said coalition troops called in air support after they were ambushed from multiple positions, including that occupied by the private security guards.

"Early yesterday, a coalition unit was on their way to their objective when they came under fire from multiple locations," said U.S. forces spokesman Colonel Gregory Julian.

"They tried to withdraw but could not and they had no alternative but to call in close air support," he said.

"Afterwards they discovered there were casualties with contract security uniforms and therefore an investigation was launched to determine what happened."

Afghanistan has suffered a marked escalation of violence this year, the bloodiest period since U.S.-led and Afghan forces toppled the Taliban in late 2001 for refusing to give up al Qaeda leaders behind the September 11 attacks on the United States.

Hundreds of civilians have been killed by foreign troops in operations against Taliban militants in Afghanistan this year, according to Afghan officials and aid groups.