Pakistan troops killed in border clashes with Afghan forces after U.S. air strike

The troops died as U.S. coalition forces in Afghanistan battled militants attacking from Pakistan, according to a security official from Pakistan.

Many Al Qaeda and Taliban militants took refuge on the Pakistani side of the border after U.S.-led forces ousted the Taliban in Afghanistan in 2001.

Gun battle: A Pakistani soldier is helped from an ambulance after the bloody clash

But the Pakistani military said the air strike in the Mohmand region, near Afghanistan's Kunar province, 'hit at the very basis of co-operation and sacrifice with which Pakistani soldiers are supporting the coalition in the war against terror'.

It added: 'Such acts of aggression do not serve the common cause of fighting terrorism.'

It was Pakistan's strongest criticism of the U.S. military since joining the campaign against terrorism.

Battle-scarred: A Pakistani man, who was injured in an attack on Pakistan's Mohamand Tribal agency along the Afghanistan border

Frustration is rising in Kabul and among Western forces in Afghanistan over Pakistani efforts to negotiate pacts to end militant violence on its side of the border.

Nato says such deals lead to more violence in Afghanistan.

The U.S. military said it had co-ordinated the artillery and air strike with Pakistan, but is investigating further.