An Afghan policewoman has killed an American adviser at Kabul's police headquarters.

Kabul's deputy police chief Mohammad Daoud Amin said an investigation was under way to determine whether the killing was intentional or accidental.

It was not known whether the victim was a military or civilian adviser. The Nato military command said it was looking into reports of the shooting but had no independent information.

At least 53 international troops have been killed by Afghan soldiers or police this year and a number of other assaults are still under investigation.

Nato forces, due to mostly withdraw from the country by 2014, have speeded up efforts to train and advise Afghan military and police units before the pull-out.

More than 50 international troops have been killed by Afghan soldiers or police this year, and a number of other assaults - insider attacks as they are known - are still under investigation.

Nato forces, due to mostly withdraw from the country by 2014, have speeded up efforts to train and advise Afghan military and police units before the pullout.

The surge in insider attacks is throwing doubt on the capability of the Afghan security forces to take over from international troops and has further undermined public support for the 11-year war in Nato countries.

More than 50 Afghan members of the government's security forces also have died this year in attacks by their own colleagues.

Taliban militants claim such attacks reflect a growing popular opposition to both foreign military presence and the Kabul government.