The US military said in a statement that coalition troops had come under fire from "anti-Afghan forces" in a wooded area near the Pakistan border.
The statement said artillery and air strikes had been co-ordinated with Pakistan, but that the incident was being investigated.
A spokesman for a pro-Taleban militant group in Pakistan said it had launched an attack on US and Afghan army troops trying to set up a border control post.
"We launched an attack on them from several sides and caused serious harm - and then the US and Nato forces began a series of air strikes," said the spokesman, Maulvi Umar.
There is increasing anger in Pakistan at US strikes on its territory which have killed more than 50 people this year, says the BBC's Barbara Plett in Islamabad.
Both US forces and Nato-led coalition forces are operating in Afghanistan, with Nato focused mainly on peacekeeping and reconstruction and the US troops working more directly to combat militant activity.
Funerals have been held for the 11 soldiers who died
Taleban fighters have a strong presence in the border areas of the tribal districts and local administrators have little power there, although security forces keep a presence on the border.
There is rising frustration among the Afghans and foreign troops at Pakistani efforts to negotiate peace deals with pro-Taleban militants on its side of the border.
Afghan and US-led forces accuse Islamabad of not doing enough to deny Taleban militants a hiding place in Pakistan's tribal areas and to stop them from infiltrating the border into Afghanistan.
They are worried that the Pakistan government's recent peace talks with the militants there will only give the Taleban more room for manoeuvre.
Pakistan denies the accusations, saying it has lost about 1,000 soldiers fighting militants in the tribal border areas.
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