Two Pakistani soldiers were reported wounded in an exchange of fire with two NATO helicopters near the Afghan border.
Pakistani forces in the North Waziristan tribal region fired at the NATO helicopters, which returned fire, wounding the Pakistani soldiers.
But it was unclear whether the helicopters had crossed the Pakistani border.
A Pakistani military statement said the army lodged a "strong protest" with NATO, saying the alliance "violated Pakistan airspace" and demanded a meeting with officials of the alliance.
Earlier in the day, a military spokesman, Azmat Ali, told RFE/RL's Radio Mashaal that the helicopters did not cross the Pakistani border.
A Western military official in Afghanistan said coalition forces were aware of the incident but could not confirm whether the helicopters had crossed into Pakistani airspace.
The incident comes after relations between the United States and Pakistan were severely strained by the killing of Osama bin Laden by U.S. special forces in a commando raid in Pakistan in early May.
compiled from agency reports