A foreign airstrike killed nine Afghan policemen in western Afghanistan overnight after a clash in which both sides mistook the other for Taliban militants, Afghan officials said on Sunday.

The incident is certain to reinforce Afghan perceptions that international troops do not take enough care to avoid hurting innocents and comes after a string of mistakes that Afghan officials say killed dozens of civilians.

Clashes broke out between Afghan police and international troops in the Anar Dara district of Farah province, with both sides thinking the other were Taliban militants, the deputy provincial governor Mohammad Younus Rasuli said.

"Apart from the nine police who were killed, three other of them have gone missing. We do not know if they are under the rubble or their bodies can not be found," Rasuli told Reuters.

"U.S. soldiers have surrounded the site and we have send a delegation about the incident," he added.

The foreign troops called in airstrikes on the police post that killed nine policemen and wounded four others, including the district police chief, he said.

Both NATO's International Security Assistance Force (ISAF) and the separate U.S.-led coalition force in Afghanistan said they were aware of an incident in Farah, but could not confirm any details of what had happened.

Meanwhile, ISAF troops accidentally killed four Afghan civilians in a mortar attack overnight in the Barmal district of the eastern province of Paktika, close to the Pakistani border, the force said.

"An ISAF unit fired two mortar rounds, which landed nearly 1 km (0.6 miles) away from the intended target," ISAF said in a statement. "Shortly afterwards wounded civilians presented themselves for treatment at an ISAF base, and a helicopter medical evacuation mission was immediately launched to assist." There were another three unconfirmed deaths, it said, and four civilians were also wounded in the attack.

"ISAF deeply regrets this accident, and an investigation as to the exact circumstances of this tragic event is now under way," the statement said.

Afghan officials say U.S.-led coalition airstrikes killed more than 60 Afghan civilians, many of them women and children, in the east of the country earlier this month. U.S. forces have launched investigations into the incidents.

While NATO's International Security Assistance Force (ISAF) denied reports by Afghan officials that it killed 50 civilians in the west of the country last week, many Afghans believe foreign troops do not take enough care when launching attacks.

Mostly U.S. troops in the east of the country are battling a sharp rise in violence along the Pakistani border this year, which they put down to de facto ceasefires between militants and the military in Pakistan's tribal belt which allow the Taliban sanctuaries from which to launch more strikes into Afghan soil.