Afghan officials say the police were not informed of the US raid
Six Afghan policemen and a civilian have been killed in air strikes after a clash with US-led special forces in the south of the country, US officials say.
Thirteen other policemen were wounded in the Zabul province attack, they say.
Zabul's deputy police chief said an investigation had begun into what the US military said was a case of mistaken identity by both parties.
Correspondents say so-called friendly fire incidents between US or Nato and Afghan forces happen frequently.
Gulab Shah Alikhail, Zabul's deputy governor, told the AP news agency that US special forces were carrying out an attack against militants near a police checkpoint on the outskirts of provincial capital Qalat.
The police, thinking it was a Taleban attack, opened fire, he said. Then a helicopter fired on the security post and destroyed it, he said.
Officials say that the US special forces did not inform the police they were going to the village because of fears their plans could be passed on to the Taleban by sympathetic officials.
A US military statement confirmed the Americans returned fire on the police but only later learned their identities.
"Coalition forces deeply regret the incident of mistaken fire," US military spokesman Col Jerry O'Hara said.
"Initial reports indicate this was a tragic case of mistaken identity on both parts."
The US statement said that the target of Wednesday's raid was a militant commander "known to co-ordinate attacks against coalition forces along Highway One".
The road is Afghanistan's main highway that circles the country. US military officials say that the commander was killed in the operation.
The BBC's Martin Patience in Kabul says that this is not the first time the US military has mistakenly killed Afghan security forces.
In October, at least nine Afghan soldiers were killed in the eastern province of Khost by American airstrikes - after being mistakenly identified as Taleban insurgents.