Kabul: NATO said its aircraft entered Pakistani airspace early Thursday in self defence and killed "several armed individuals" after air crews believed they had been fired at from the ground.
After striking what was believed to be an insurgent group, "the aircraft received what the crews assessed as effective small arms fire from individuals just across the border in Pakistan," NATO`s International Security Assistance Force (ISAF) said in a statement.
"Operating in self defense, the ISAF aircraft entered into Pakistani airspace killing several armed individuals," it said.
Pakistan shut down the main land route for NATO supplies into Afghanistan after officials accused NATO of killing Pakistani troops in the fourth cross-border attack this week.
A Pakistan military spokesman in a statement said two helicopters from Afghanistan used cannon fire against an outpost of the Frontier Corps located 200 metres (650 feet) inside Pakistan.
Troops present at the post manned by six soldiers "retaliated through rifle fire to indicate that the helicopters were crossing into our territory," the spokesman said.
"Instead of heeding to the warning, helicopters went to fire two missiles, destroying the post. As a result, three FC soldiers have embraced shahadat (martyrdom) and three have been injured."
ISAF said it had been informed by Pakistani military officials that members of their border forces had been hit by coalition aircraft and said a review was under way "to verify the exact location of the two engagements and the facts".
"Both sides have in mind that it is the insurgents, operating on the border between Pakistan and Afghanistan and violating the territorial sovereignty of both countries, that we are focused on fighting," ISAF said.
NATO and the Pakistani government said earlier they were investigating the incident in the Kurram district of Pakistan`s tribal belt, which Washington has branded an al Qaeda headquarters and hub of militants fighting in Afghanistan.
The region is being targeted by a record number of US drone strikes and was reportedly where al Qaeda hatched a recent plot to attack cities in Britain, France and Germany uncovered by Western intelligence agencies.
"We have suspended NATO supply trucks for the time being due to security reasons," an official in Pakistan`s Frontier Corps paramilitary unit told AFP in the northwestern city of Peshawar on condition of anonymity.
Khyber is on the main NATO supply route into Afghanistan, where more than 152,000 US and NATO forces are fighting a nine-year Taliban insurgency.
Pakistan has condemned cross-border air strikes by NATO helicopters pursuing militants into its territory. NATO has said previously it has the right to self-defence.
ISAF said its crews had "observed what it believed was a group of insurgents attempting to fire mortars at a coalition base in the border area of Dand Patan district, Paktiya province.
"An ISAF air weapons team was called to provide fire support and engaged the suspected insurgents` firing position, located inside Afghanistan along the border area.
"ISAF aircraft did enter into Pakistani airspace briefly as they engaged this initial target," it said.
The US presence in Afghanistan and US drone strikes in the tribal belt are the subject of fierce criticism and suspicion in Pakistan.