North Korea said today it had arrested a South Korean spy and was investigating him on charges of espionage in a rare report of the capture of a secret agent by either of the rivals.

The man was arrested in the North Korean capital, Pyongyang, and had confessed to entering the country illegally, the North's Ministry of State Security said in comments carried by the North's official KCNA news agency.

"An initial investigation indicates that he was engaged in anti-DPRK espionage and plot-breeding activities in a third country bordering the DPRK for nearly six years, while disguising himself as a religionist," KCNA said, referring to the North's official name, the Democratic People's Republic of Korea.

KCNA did not identify the suspect nor the "third country". It did not elaborate on what it meant by "religionist".

"He entered the DPRK to rally dishonest elements within the boundary of the DPRK and use them for undermining the stability of the social system," KCNA said. It gave no more details.

An officials for the South's National Intelligence Service dismissed the report as "groundless" and said the agency had no further comment.

Public announcements of the arrests of secret agents by either the South or the North have become rare despite the consistently high level of espionage activities conducted by the rivals since the end of the 1950-53 Korean War.

North Korea has prosecuted U.S. citizens for conspiring to undermine its security and is holding a Korean-born American missionary on a 15-year hard labour sentence.

North and South Korea have been in a standoff after a new attempt to promote dialogue broke down in September amid acrimony and doubts on both sides about the sincerity of the other on engagement aimed at easing tension.

Months of hostile rhetoric early this year pushed tension to some of the highest levels in years with North Korea, which has conducted nuclear tests, threatening a nuclear strike on the United States and South Korea.