ISLAMABAD (AFP) - A United States citizen was arrested at Islamabad airport on Friday for attempting to board a plane carrying ammunition, Pakistan officials said, in the second such case since May.

The man, who was identified by police and airport security as a US security trainer, was passing through security checks at Benazir Bhutto International Airport when he was caught with 15 rounds for a 9mm handgun.

Mr Muhammad Fayaz, a police officer deputed at the airport, said: "The Airport Security Force (ASF) has handed over to us a US national after recovering a magazine and 15 bullets from him.

"The bullets and magazine were discovered through a scanning machine."

An airport security official told AFP that the man was set to travel by Etihad flight EY-232 from Islamabad to Abu Dhabi and onward to the United States.

"He is residing in the US embassy as a security trainer," the official said.

A spokesman for the US embassy was not immediately able to provide confirmation of the arrest.

The incident came two months after US FBI agent Joel Cox was arrested at Karachi airport and detained for a few days for carrying the same type and amount of ammunition.

The case against him was later dropped after the Pakistani government said he was authorised to carry the rounds.

The agent's lawyer also submitted a copy of a letter from the US embassy showing that he was on a mission and was allowed to carry the ammunition.

US-Pakistan ties have improved markedly since almost collapsing in 2011 amid a series of crises, including the US raid in Pakistan that killed Al-Qaeda chief Osama bin Laden - which Islamabad branded a violation of sovereignty.

The fatal shooting of two men by CIA contractor Raymond Davis in the eastern city of Lahore in January 2011 also sparked a diplomatic crisis between the two "war on terror" allies.

A Pakistani court eventually freed Davis following the payment of US$2 million (S$2.5 million) in blood money to the families of the dead men.