An ex-FBI agent, Donald Sachtleben, has pleaded guilty to leaking national defence information to a reporter about an al-Qaeda bomb plot and to child pornography charges.

The plea agreement calls for him to be sentenced to a total of 140 months in prison – 43 for the national security offenses and 97 for the child pornography charges.

In May 2012, the Associated Press, through the help of Sachtleben, published an article describing a successful effort to disrupt a plot by Yemen-based al-Qaeda militants to bomb a US-bound airliner.

The ex-FBI agent according to a court document signed, said he disclosed secret information to a reporter on May 2, 2012.

Nine days later he was arrested on separate charges of trading images of child pornography online, having been identified during an investigation into another man accused of possession of child pornography and after seizing phone records of the news agency.

Official said the information jeopardised an international intelligence operation and put lives at risk.

“It put the American people at risk, and that is not hyperbole,” US Attorney-General, Eric Holder said, ranking the national security disclosure as among the most serious he had ever seen.

“This unauthorized and unjustifiable disclosure severely jeopardized national security and put lives at risk,” deputy Attorney-General, James Cole said in a statement.

He added that: “To keep the country safe, the department must enforce the law against such critical and dangerous leaks, while respecting the important role of the press.”

The Associated Press declined to comment on its relationship with Sachtleben, but reported he had pleaded guilty “to revealing secret information for an Associated Press story”.

Sachtleben worked for the FBI as a bomb technician from 1983-2008 and held top secret security clearance.