A US citizen has been detained in the Pakistani city of Peshawar after he was allegedly discovered with bullets in his luggage at the airport, police say.
Officials say he was set to fly to the Pakistani capital, Islamabad.
They are checking reports he might have diplomatic immunity. US authorities say they are investigating.
US-Pakistan ties deteriorated sharply after the killing of Osama Bin Laden by covert US special forces in north-west Pakistan last May.
Relations were already tense after an earlier incident where a CIA contractor killed two Pakistani men in Lahore. Raymond Davis was detained by Pakistani police, despite claims that he had diplomatic immunity.
He was eventually released after a "blood money" deal was made with the families of the two victims. But the case ignited anti-American feeling across Pakistan and prompted angry demonstrations.
"He has been detained at the airport and is under interrogation," senior police officer Tahir Ayub told Reuters new agency, of the man detained in Peshawar.
"We are in contact with the foreign office in Islamabad to check about his background and tell us if he has diplomatic immunity," Mr Ayub said.