North Korea says it has detained a 24-year-old US citizen who demanded asylum after arriving in the country on a tourist visa.

The North's official KCNA news agency says Matthew Todd Miller has been in custody since he apparently ripped up his visa at immigration and demanded asylum on April 10.

According to KCNA, Mr Miller tore his visa up into pieces and shouted that "he would seek asylum" and had come to North Korea "after choosing it as a shelter".

Such "rash behaviour" constituted a "gross violation" of North Korean law, the news agency said. The report said he remained in detention and was under investigation.

The announcement came as US president Barack Obama wrapped up the first part of a two-day visit to South Korea.

US State Department spokeswoman Jen Psaki said Washington was aware of the reports of the arrest of a US citizen, but she had no additional information.

"We have been in touch with the embassy of Sweden about these reports," she told journalists, adding that "there's no greater priority to us than the welfare and safety of US citizens."

Washington has no diplomatic ties with Pyongyang, and the Swedish embassy there usually acts on its behalf in cases involving US citizens.

US again presses N Korea for Bae's release

North Korea is currently holding another US citizen, Kenneth Bae, described by a North Korean court as a militant Christian evangelist.

Mr Bae was arrested in November 2012 and sentenced to 15 years' hard labour on charges of seeking to topple the government.

US efforts to secure Mr Bae's release have so far been unsuccessful, but Ms Psaki said he had been visited by Swedish embassy representatives 11 times since his arrest, the last time being on April 18.

"We remain deeply concerned about his heath," Ms Psaki said, adding "we continue to urge North Korean authorities to grant Mr Bae special amnesty and immediate release on humanitarian grounds".

Last month, North Korea freed 75-year-old Australian missionary John Short, a month after his arrest in Pyongyang.

Mr Short was accused by Pyongyang of committing a crime by distributing bible tracts at a Buddhist temple in the capital on the birthday of former North Korean leader Kim Jong Il.

North Korea said it had decided to deport Mr Short, partly in consideration of his age, and issued what it said was a picture of a handwritten note from him apologising for his act.

North Korea has in the past freed detained Americans after visits from high-level emissaries.

In 2011, a US delegation led by Robert King, the US special envoy for North Korean human rights issues, secured the release of Eddie Jun Yong-Su, a California-based businessman, detained for apparent missionary activities.

In 2010 former US president Jimmy Carter won plaudits when he negotiated the release of American national Aijalon Mahli Gomes, sentenced to eight years of hard labour for illegally crossing into the North from China.

On another mercy mission a year earlier in 2009, former president Bill Clinton won the release of US television journalists Laura Ling and Euna Lee, jailed after wandering across the North Korean border with China.

AFP