KABUL : Afghanistan's NATO-led force has launched an investigation into the accidental killing of five Afghan policemen in a US air attack during an overnight operation, the force said on Thursday.

Accidental casualties caused by Western air strikes are a major source of friction between President Hamid Karzai and his international allies.

The Wednesday night incident, in which two policemen were wounded, comes as the United States is attempting restart stalled talks with Karzai's government on the size of a military presence after most international troops leave next year.

Afghan special forces called for the air support during a clash with a group of Taliban fighters at a police checkpoint, said Ahmad Zia Abdulzai, a spokesman for the governor of the eastern province of Nangarhar, where the incident happened.

"A US aircraft engaged, inadvertently killing five Afghan National Police members and wounding two," said Master Sergeant Bryan Gatewood, a spokesman for the NATO-led force.

"An investigation is ongoing to determine additional details of this unfortunate incident."

Afghan security forces do not possess a significant air force and foreign air power is considered critical to cover the mountainous regions near the Pakistani border, which have long been insurgent hot spots.

But accidental Afghan casualties in air strikes, particularly civilians, infuriate Karzai and he had previously ordered a ban on his troops calling them in.