Two NATO helicopters on Tuesday launched a cross-border attack on a Pakistan army checkpoint in a restive tribal region, wounding two security personnel, local security officials said.
The attack took place in Wacha Bibi, 50 kilometres (30 miles) west of Miranshah, on Tuesday morning with two NATO helicopters entering Pakistani airspace from Afghanistan and attacking the checkpoint, they said.
"Two NATO helicopters committed the airspace violation and shelled an army checkpoint, injuring two soldiers," a senior security official told the media.
The incident came after the United States launched a raid from Afghanistan on May 2 that killed Al-Qaeda leader Osama bin Laden near Islamabad, in an attack that embarrassed and angered the Pakistani military and leadership.
A spokesman for the international military alliance in Afghanistan said ISAF "had reports of a possible incident. We are looking into it".
"I can't confirm any of the details" of press reports," Lieutenant-Colonel John L. Dorrian, said.
The Pakistani military often accuses a NATO force of violating of Pakistan's air space, in the hunt of Taliban who launch attacks across the border.
Pakistan temporarily shut the main land route for NATO supplies into Afghanistan last September after officials accused NATO of killing Pakistani troops in a cross-border attack.
The region is being targeted by a record number of US drone strikes, the number of which has doubled in the last year, with more than 100 strikes killing over 670 people, according to an AFP tally. The CIA says the covert programme has severely disrupted Al-Qaeda's leadership.
US drone strikes inflame anti-American feeling in Pakistan, which has worsened since a CIA contractor shot dead two Pakistani men in a busy Lahore street in January.
Two such US drone strikes targeting a militant compound and a vehicle in Pakistan's lawless tribal district of North Waziristan on Monday killed at least nine people.
Washington considers Pakistan's tribal belt the global headquarters of Al-Qaeda and the most dangerous place on Earth, where Taliban and other militants plot attacks on US troops in Afghanistan and on Western targets.