Image copyright AP Image caption FBI officials tracked reporter James Rosen's comings and goings from the state department building

A US state department official has pleaded guilty to passing classified information on North Korea to an American journalist.

Stephen Kim, 46, now faces a 13-month sentence as part of a plea deal.

The case began after Fox News journalist James Rosen published a story in June 2009 about US intelligence on North Korea.

It is one in a series of recent prosecutions against unauthorised leaks from government sources.

The Obama administration has prosecuted more leakers than all previous US administrations combined.

Kim, who was a former senior adviser for intelligence to an assistant secretary of state, will be sentenced on 2 April if a judge accepts his plea.

Journalist 'co-conspirator'

The information found in Rosen's article came from an intelligence report sent to several officials, including Kim, on the same morning Rosen's story was published, according to prosecutors.

Image copyright Reuters Image caption North Korea, led by Kim Jong-un (centre), is one of the world's most secretive societies

Rosen reported US intelligence officials had warned the president and other administration advisers that North Korea would respond to a United Nations resolution condemning nuclear tests with another nuclear test.

Kim's lawyer, Abbe Lowell, had previously said the information his client was accused of passing on was neither eye-catching nor truly secret.

"The US government has a very legitimate interest in protecting the real national security secrets of our nation," Mr Lowell said.

"The problem is that so much information is improperly classified that you can't distinguish between the real national secrets and those that just have a stamp on it that says secret."

The case had also become controversial because an FBI affidavit characterised Rosen as a "co-conspirator" with Kim, and suggested there was probable cause to believe the reporter committed a crime.

Rosen has not been charged, but authorities used the affidavit as legal cover to search Rosen's private emails and to track his comings and goings from the state department.