NATO said at least two Afghan soldiers died along with the eight Americans.
Afghan provincial authorities said they had lost contact with scores of Afghan policemen after the day-long attack and did not know whether they were dead or alive.
The fighting took place in Nuristan province's Kamdesh district in high mountains along the eastern border with Pakistan yesterday but was not reported until today.
The battle showed the ferocity of the insurgency in a part of the country that US forces have decided to abandon after years of heavy fighting.
The troops had already announced plans to withdraw from the area as part of commander General Stanley McChrystal's strategy to focus his forces on population centres.
Militia from a local mosque and a nearby village launched the attacks on two joint NATO and Afghan outposts, the NATO-led International Security Assistance Force said.
As the battle raged, foreign troops sent in F-16 jet fighters and Apache helicopter gunships to help the forces caught in the battle, a spokeswoman for the US military said.
The province's deputy police chief said the fate of an entire 90-strong police force in the Kamdesh district was unknown.
The attack was the deadliest for US forces since nine were killed in a July 2008 battle in nearby Kunar province, which the US military is investigating as a debacle that will teach its forces how to understand the demands of combat in Afghanistan.
US forces have suffered some of their worst casualties in the east, where they have tried to control remote mountain passes used by Taliban fighters as infiltration routes from Pakistan.
