James Bays, Al Jazeera's correspondent in Afghanistan, said it was one of the worst incidents of mistaken fire in Afghanistan.
He said most of the Afghan police officers were killed and injured when a helicopter gunship arrived on the scene and started firing rockets.
A US forces spokesman said a combined coalition-Afghan force on a mission against a Taliban safehouse was fired on first and responded to the attack.
Afghan police manning the isolated checkpoint on a barren stretch of desert said the US troops fired first.
Esanullah, the commander of the checkpoint, said eight police were killed and four wounded.
|The police commander said US forces should |
have seen the sirens on the trucks [Reuters]
"The Americans came close to our checkpoint with the lights of their vehicles off.
"We shouted at them to stop, but they didn't, and they opened fire on us."
The police fired 49 of their 50 rocket-propelled grenades at the US forces, and called for assistance from reserve police during the three-hour battle, said Esanullah.
The sirens on the police trucks - which were riddled with bullet holes in the battle - should have been clearly visible to the US forces, he said.
The US military said there was nothing "to indicate the opposing force was friendly. The individuals who fired on coalition forces were not in uniform". It said it knew of no US injuries.
Khan Mohammad, one of the policemen at the post, said: "I thought they were Taliban, and we shouted at them to stop, but they came closer and they opened fire."
"I'm very angry. We are here to protect the Afghan government and help serve the Afghan government, but the Americans have come to kill us."
Poor co-ordination blamed
Karzai's spokesman, Karim Rahimi, said the incident underscored why the president has repeatedly called for increased co-operation between Afghan and foreign troops.
|"I'm very angry. We are here to protect the Afghan government and help serve the Afghan government, but the Americans have come to kill us" |
Khan Mohammad, police officer
"The police forces were not aware of the coalition's operation," said Rahimi.
"The police checkpoint in the area thought that they were the enemy, so police opened fire on the coalition, and then the coalition thought that the enemies were firing on them, so they returned fire back."
The US military said "prior to the onset of the mission, coalition forces co-ordinated with officials to ensure no conflicting operations were occurring in that area", without specifying if Afghan police had been informed.
Night raids by US special forces in particular have been criticised for causing civilian casualties.
The International Red Cross has said that Afghan civilians were increasingly paying the price of the fighting between the Taliban and international troops.
"The humanitarian situation in Afghanistan is worse now than it was a year ago," Pierre Kraehenbuehl, director of operations for the International Committee of the Red Cross, said.