The man identified as William Johns, who worked as a security trainer at the US embassy in Islamabad, was detained earlier today at the Benazir Bhutto International Airport while carrying 15 bullets of 9 mm calibre and a magazine.
After initial investigation by Airport Security Force, he was handed over to police.
Abid Ahmad, a police official at Airport Police Station, told PTI that a case was registered against Johns for carrying illegal weapons.
The accused had confessed to carrying the ammunition by mistake. He was planning to board a Dubai-bound Etihad Airlines flight on way to the US.
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It is the second incident of arrest of a US embassy official with ammunition.
In May, Joel Cox, an FBI agent, was arrested with pistol bullets from Karachi airport.
He was later set free when the US embassy showed a letter proving that he was authorised to carry weapons.
US relations with Pakistan have been fraught since CIA contractor Raymond Davis shot two men dead in Lahore in 2011.