Seoul, South Korea: North Korea detained two Americans for illegally crossing its border and is investigating them, the communist country's official news agency said on Saturday.

The arrests come at a sensitive time, with the North planning on firing a satellite-equipped rocket into space in early April - a launch some fear will be a cover for testing missile technology.

The North also is locked in a standoff with regional powers over its nuclear programme, and earlier this week expelled five US groups that distribute much-needed food aid in a country where the World Food Programme says millions are going hungry. It has also repeatedly shut its southern border in recent days to protest joint US-South Korean military exercises.

The two female US journalists were arrested on Tuesday while illegally intruding into the territory" of North Korea after crossing the border with China, the Korean Central News Agency said.

Authorities were investigating, KCNA said. The brief dispatch gave no further details.

A US Embassy spokesman in Seoul said he had no further information. He asked not to be named, citing the sensitivity of the issue.

South Korean media and a South Korean missionary identified the two Americans as Laura Ling and Euna Lee, reporters for former Vice-President Al Gore's San Francisco-based media outlet Current TV.

State Department officials said Washington is in contact with North Korea about the detentions. US Secretary of State Hillary Rodham Clinton "is engaged on this matter right now," spokesman Robert A. Wood told reporters on Friday. "There is a lot of diplomacy going on." He did not elaborate.

The two reporters were in the border area with a male cameraman and their guide as part of a reporting assignment on North Korean refugees.

The journalists headed to the Chinese city of Yanji, across the border from North Korea's far northeastern corner, where they planned to interview women forced by human traffickers to strip for online customers, according to Reverend Chun Ki-won of the Seoul-based Durihana Mission, a Christian group that helps defectors.

Chun said Ling and Lee contacted him three months ago asking for help organising a trip to China.

They also planned to meet with children of defectors, Chun said. Many children who grow up on the run in China live in legal limbo, unable even to attend school, according to a 2008 Human Rights Watch report.

The journalists and cameraman Mitch Koss were following a guide across the frozen Tumen River early on Tuesday morning when North Korean soldiers armed with rifles approached them from a half-hidden guard post, the Chosun Ilbo newspaper said. It cited activists working with North Korean refugees in China and other unidentified sources.

Koss and the guide pushed the North Korean soldiers away and ran back toward China, but Ling and Lee were caught.

Koss and the guide were later seized by Chinese border guards and sent to the Chinese Public Security Bureau. Their whereabouts remain unclear.