Pakistan's Prime Minister Yusuf Raza Gilani has condemned an air strike by Afghanistan-based US forces that Islamabad says killed 11 of its troops.
The incident took place inside Pakistan, near the border with Afghanistan.
The US military confirmed it had used artillery and air strikes after coming under fire from "anti-Afghan" forces.
The incident comes as relations between the US and Pakistan militaries have been hit by mounting tensions.
The soldiers' deaths occurred overnight at a border post in the mountainous Gora Prai region in Mohmand, one of Pakistan's tribal areas, across the border from Afghanistan's Kunar province.
Eight Taleban militants were also killed in the clashes, a Taleban spokesman said.
'Cowardly act'
If the 11 deaths are confirmed, it would be the worst incident of its kind since US and Nato-led forces began fighting militants in Afghanistan in 2001.
Prime Minister Gilani condemned the deaths, telling parliament: "We will take a stand for the sake of this country's sovereignty, for the sake of its dignity and self-respect".
"We do not allow our territory to be used. We completely condemn this, and will take it up through the foreign office."
Pakistan's military called it a "completely unprovoked and cowardly act".
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
