•  Pakistan halts NATO supplies following deadly strike

NATO described the killings as a "tragic unintended incident" and said an investigation was underway. A Western official and an Afghan security official who requested anonymity said NATO troops were responding to fire from across the border.

It's possible both explanations are correct: that a retaliatory attack by NATO troops took a tragic, mistaken turn in harsh terrain where identifying friend and foe can be difficult.

"All of this is extremely murky and needs to be investigated," said a US official in Washington, who spoke on condition of anonymity. "Our goal today is ... that the investigation gets mounted in a way that is confidence-building on all sides."

Militants often attack from Pakistani soil or flee after combat across a porous border that NATO-led troops, under their United Nations mandate, cannot cross.

Reuters