Afghan 'policeman' kills six US troops
A gunman in an Afghan police uniform has killed six US service members in eastern Afghanistan, officials say.
The man opened fire during a training mission in Pachir Wagam district, Nangarhar province, said Nato. He was also killed in the incident.
US officials later confirmed that all six were Americans, but declined to give further details.
The Taliban issued a statement saying it was responsible for the killings, AP news agency reported.
Spokesman Zabiullah Mujahid said the gunman had joined the border police in order to kill foreign soldiers.
"Today he found this opportunity and he killed six invaders," he said.
Nato said the incident was being investigated.
"An individual in an Afghan border police uniform turned his weapon against International Security Assistance Forces (Isaf) during a training mission today, killing six service members in eastern Afghanistan," Monday's statement said.
"The individual who fired on the Isaf forces was also killed in the incident."
Regional police commander Gen Aminullah Amerkhail said the area was remote and telephone lines were not working, making it hard to get accurate information on the incident.
Taliban insurgents have previously dressed as police to carry out attacks.
Only at the weekend, two suicide bombers in police uniform killed 12 police officers in south-eastern Paktika province.
The Afghan interior ministry says new recruits are checked to make sure they do not have a criminal record - and their village elders have to vouch for their good character. So there is a vetting procedure for recruits to the Afghan forces - but it is not extensive.
In addition, so many new recruits are being taken on, it is doubtful how thorough the checks can be. There are now some 260,000 members of the Afghan security forces - 160,000 were trained in the last year. The US alone is spending some $11bn (£7bn) a year on training the Afghan security forces.
This latest incident of a soldier or police officer in Afghan uniform apparently firing on Nato will raise fresh questions about the extent to which the Taliban may have infiltrated the Afghan forces, their loyalty and their reliability.
Five British soldiers were shot dead in November last year in Helmand province, by an Afghan policeman, possibly a militant infiltrator, who then escaped.
Isaf is training and mentoring Afghanistan's security forces, but there have also been several incidents of Afghan soldiers firing on foreign troops.
Nato said earlier this month it was investigating Taliban claims that an Afghan soldier had shot dead foreign troops in the south of the country.
In July a renegade Afghan soldier shot and killed three British army Gurkhas at a base in Helmand province.
A week earlier an Afghan soldier killed two American contractors inside a military base in northern Afghanistan.
Nato's exit strategy for Afghanistan involves progressively handing over to the local security forces.
But the BBC's Paul Wood in Kabul says this latest incident will raise questions again about the loyalty and reliability of those forces and the extent to which they could have been infiltrated.