Pakistani troops fired on U.S. helicopters patrolling eastern Afganistan Thursday, the Pentagon said, adding that it expects an explanation from Pakistani officials.

Pakistan said it was their own troops and foreign helicopters that opened fire in the Afghan border incident.

The Associated Press originally reported that Pakistani troops fired at NATO helicopters, but no damage was reported.

The Pentagon said U.S. helicopters "at no point" crossed into Pakistan's airspace and that they were shot at near the Tanai district of Khost province.

"ISAF helicopters received small-arms fire from a Pakistan military checkpoint along the border near Tanai district, Khost, September 25 while conducting routine operations in Afghanistan," the International Security Assistance Force said in a statement, according to AFP.

The shooting follows a number of alleged incursions by U.S. aircraft into Pakistan from Afghanistan in recent days. A drone believed to be operated by the CIA crashed inside Pakistan on Wednesday.