Joel Cox was about to board a domestic flight to Islamabad at the Jinnah International Airport on Monday evening when the Airport Security Force (ASF) discovered 15 bullets of 9mm caliber and a pistol clip in his baggage.
He was detained by the ASF before being handed over to the airport police station where a case was lodged against him.
Yesterday, Cox appeared in court on charges that he had violated local anti-terrorism laws that prohibit carrying firearms or ammunition aboard commercial flights, which carries a maximum sentence of 14 years in jail.
He was later remanded to police custody till May 10. "We are aware of the situation that has been reported and we are coordinating with Pakistani authorities to resolve this matter," said Meghan Gregonis, US Embassy spokesperson in Islamabad.
US media reports have identified him as an FBI agent who was in Pakistan to allegedly train policemen.
US officials in Washington confirmed that the agent, who is assigned to the FBI Miami Field Office, was in Pakistan on temporary duty, the Washington Post reported.
American diplomatic sources said an official statement that could identify the organisation that Cox belongs to is likely to be issued later.
Karachi police officials have denied knowledge of any such training programme and said Cox was on a 90-day 'visit' visa. Dressed in a dark trouser and striped shirt, Cox was shown on TV channels here being escorted by Pakistani security officials.