NATO helicopters have attacked a border post, Pakistani security officials say, killing at least 24 soldiers and injuring more than a dozen.
Authorities suspended NATO supplies to landlocked Afghanistan in protest.
The pre-dawn attack occurred in Mohmand agency, one of the seven Pakistani tribal districts from where US officials say militants infiltrate into Afghanistan to target NATO-led forces.
A military spokesman said NATO helicopters had targeted the Salala border post manned by paramilitary troops.
An official at the paramilitary Frontier Corps headquarters in Peshawar, the capital of north-western Khyber-Pakhtunkhwa province, said initial reports showed 22 soldiers and two officers were killed and 15 troops were wounded.
"This is an attack on Pakistan's sovereignty and such attacks will not be further tolerated," said provincial governor Masood Kausar, who also heads the civilian government in seven tribal districts.
The Salala border post is on a hill along a route used by militants often to enter into Afghanistan, said an intelligence official who also spoke on condition of anonymity.
The airstrike triggered a wave of anger in the country where anti-American sentiments already run high.
Condemnation also came from opposition political parties.
Geo Television reported that Pakistan's acting ambassador in Washington, Iffat Gardezi, has registered a strong protest with the US State Department over the attack.
Jamil Khan, a senior security official in Khyber agency, said trucks carrying supplies for NATO in Afghanistan had been stopped from crossing the border.
"So far 50 trucks have been turned back," he said.
The main route for more than 50 per cent of supplies - from fuel to food and weapons - for NATO forces in Afghanistan runs through Khyber district.
NATO-led forces confirmed that a clash with Pakistani troops had occurred, but withheld comment pending investigation.
"We are aware that there has been an incident in the border region," Lt Commander Brian Badura, spokesman for the International Security Assistance Force, said.
"Right now we are gathering details and starting to investigate what happened."
Authorities suspended NATO supplies to landlocked Afghanistan in protest.
The pre-dawn attack occurred in Mohmand agency, one of the seven Pakistani tribal districts from where US officials say militants infiltrate into Afghanistan to target NATO-led forces.
A military spokesman said NATO helicopters had targeted the Salala border post manned by paramilitary troops.
An official at the paramilitary Frontier Corps headquarters in Peshawar, the capital of north-western Khyber-Pakhtunkhwa province, said initial reports showed 22 soldiers and two officers were killed and 15 troops were wounded.
"This is an attack on Pakistan's sovereignty and such attacks will not be further tolerated," said provincial governor Masood Kausar, who also heads the civilian government in seven tribal districts.
The Salala border post is on a hill along a route used by militants often to enter into Afghanistan, said an intelligence official who also spoke on condition of anonymity.
The airstrike triggered a wave of anger in the country where anti-American sentiments already run high.
Condemnation also came from opposition political parties.
Geo Television reported that Pakistan's acting ambassador in Washington, Iffat Gardezi, has registered a strong protest with the US State Department over the attack.
Jamil Khan, a senior security official in Khyber agency, said trucks carrying supplies for NATO in Afghanistan had been stopped from crossing the border.
"So far 50 trucks have been turned back," he said.
The main route for more than 50 per cent of supplies - from fuel to food and weapons - for NATO forces in Afghanistan runs through Khyber district.
NATO-led forces confirmed that a clash with Pakistani troops had occurred, but withheld comment pending investigation.
"We are aware that there has been an incident in the border region," Lt Commander Brian Badura, spokesman for the International Security Assistance Force, said.
"Right now we are gathering details and starting to investigate what happened."