A gunman in an Afghan police uniform has killed six service members in eastern Afghanistan, Nato forces say.
The man opened fire during a training mission in Pachir Wagam district, Nangarhar province, said Nato. He was also killed in the incident.
Local officials said they believed the incident was not premeditated, but had arisen out of "a misunderstanding".
Nato has not revealed the nationalities of the dead soldiers, but local sources said they were American.
"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. A joint Afghan and Isaf team is investigating this 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.
A spokesman for the Interior Ministry, Zemeri Bashary, said the attacker was a genuine member of the border police and not an impostor, the Associated Press reports. Nato has not confirmed this.
Taliban insurgents have dressed as police to carry out attacks before now.
Only at the weekend, two suicide bombers in police uniform killed 12 police officers in south-eastern Paktika province.
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.
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