Pakistan's military on Monday warned that the NATO airstrikes on two of its border posts that killed 24 soldiers could have serious consequences, DPA reported.
It also rejected reports that NATO acted in self-defence after its troops were fired upon from Pakistan's side of the border, challenging the Western forces in Afghanistan to produce casualties to back up the claim.
"The Pakistani posts were within 300 metres of the border and they have been identified to the NATO through reference maps," Army Spokesman Major General Athar Abbas told the BBC Urdu service.
Abbas said NATO had attacked Pakistani posts seven or eight times during the last three years, killing a total of 72 soldiers and officers and wounding 250.
He told Geo television the NATO representative in Pakistan was immediately informed after the first strike, but the attack continued for up to two hours even after Pakistan's pleas for ceasefire.
NATO expressed its condolences over the pre-dawn Saturday attack in Mohmand Agency, one of the seven Pakistani tribal districts along the Afghan border.
In Washington, White House Spokesman Jay Carney called the incident a "tragedy" and said a separate US military investigation would be held.
"The loss of Pakistani life was a tragedy. We mourn the brave Pakistani service members who lost their lives, and our sympathies go out to their families and go out to Pakistan," Carney said.
The US military's Central Command, which oversees operations in the region, said it would conduct its own investigation.
"We take this matter very seriously," he said. "And we're obviously very keen on finding out exactly what happened."
Pakistani authorities stopped NATO supplies passing through its territory to landlocked Afghanistan after the incident, and has lodged strong protests in Washington and at NATO headquarters in Brussels.
The NATO-led International Security Assistance Force (ISAF) in Afghanistan has promised a thorough investigation.
"This incident has my highest personal attention and my commitment to thoroughly investigate it to determine the facts," ISAF commander US General John R. Allen said.
Relations between Pakistan and the US, already soured by the unannounced US raid that killed al-Qaeda chief Osama bin Laden in the north-western city of Abbottabad on May 2, reached a new low after Saturday's airstrikes.
The incident triggered a wave of anger in the country, where anti-US sentiments were already running high.
Opposition political parties condemned the attack and religious groups held protest rallies on Sunday.
The country's influential lawyers' community on Monday boycotted the courts and held protest rallies in several cities.