At least 20 soldiers were killed in a NATO air strike Saturday on a Pakistani checkpost in the tribal Mohmand region near the Afghan border, security officials said.
"At least 20 troops have been killed, two of them were officers," a security official told AFP.
Local administration officials in Mohmand said the toll may go up.
Earlier the Pakistan military and local officials said NATO helicopters from Afghanistan carried out an "unprovoked" attack killing at least eight troops.
Pakistan stopped NATO supplies to Afghanistan following the attack.
"We have stopped NATO supplies after receiving orders from the federal government," Mutahir Hussain, a senior administration official in Khyber tribal region, on the Afghan border, told AFP.
"Supply trucks are being sent back to Peshawar."
The convoys have been blocked at Takhta Baig town on the outskirts of Peshawar.
In Kabul, a spokesman for NATO's International Security Assistance Force (ISAF) told AFP: "We are aware that an incident did take place. We are still in the process of gathering information."
The Pakistani military has in the past accused the neighboring NATO force of violating the country's airspace in the hunt for Taliban militants who launch attacks in Afghanistan before fleeing back across the border.
Pakistan temporarily shut the main land route for NATO supplies into Afghanistan in September last year after officials accused NATO of killing Pakistani troops in another cross-border attack in its northwest.
Security and military officials in the Mohmand tribal region, which is near the Afghan border, said an army major was among the dead in the latest incident, which happened before dawn.
A military spokesman said: "ISAF/NATO helicopters carried out unprovoked and indiscriminate firing on a Pakistani checkpost in Mohmand agency last night (early Saturday)."
The border post was in the Baizai district of the rugged tribal terrain.
Washington considers the tribal belt a hotbed of Al-Qaeda, where Taliban and other militants plot attacks on American troops -- including those in the US-led international force based in Afghanistan -- and Western interests.
Pakistani officials have also complained about an increase in cross-border rocket attacks from Afghanistan, purportedly from militants, while NATO commanders in Afghanistan say rockets have also been fired back the other way.
Masood Kausar, Governor of the northwestern Khyber Pakhtunkhwa province, strongly condemned the apparent NATO firing.
"Such cross-border attacks are unacceptable and intolerable," he said, adding the government would take up the matter at the highest level and launch a thorough investigation.
The incident threatens to further complicate ties between Washington and its terror ally Pakistan.
Relations deteriorated sharply this year over a unilateral American raid that killed Osama bin Laden in May and over accusations that Pakistani intelligence was involved in a September siege of the US embassy in Kabul.
At talks in Islamabad last month, US Secretary of State Hillary Clinton called on Pakistan to take action within "days and weeks" on dismantling militant havens and encouraging the Taliban into peace talks.
The northwestern tribal region is also the target of a record number of US drone strikes, which doubled last year from the year before, with more than 100 strikes killing over 670 people, according to an AFP tally.
The CIA says the covert program has severely disrupted Al-Qaeda's leadership, but the strikes inflame anti-American feeling in Pakistan.