North Korea said Thursday it would charge an American detained since November with crimes against the nation, amid reports he was engaged in missionary work in the hardline communist state.

A man identified as Jun Young-Su has been investigated "for committing a crime against the DPRK (North Korea) after entering it", the official news agency said.

"He admitted his crime in the course of investigation," it said, adding officials are preparing to bring charges.

The agency said Washington had been told of the arrest and Swedish diplomats, who represent US interests in Pyongyang, had been given access to the detainee.

The US State Department disclosed the detention Tuesday and confirmed Swedish diplomats had been given access. It gave no details of the individual but appealed for the person's release on humanitarian grounds.

A source in Seoul familiar with North Korean affairs identified the man as a Korean-American businessman in his 60s who was detained for involvement in missionary work.

The man, who attends a church in Orange County, California, has been travelling frequently to the North disguising himself as a trader, the source said.

Yonhap news agency, JoongAng Daily newspaper and other South Korean media outlets gave similar accounts.

The source told AFP the man was arrested right after the North's bombing of a South Korean border island last November which killed four people and sent cross-border tension soaring.

"It looks like the North had been watching the missionary for quite some time and arrested him for a political bargaining chip at what it thought was a suitable time to take advantage of him," the source said.

It was the third apparent case in less than a year of a US Christian activist being detained in the North. Rights groups say Pyongyang severely restricts freedom of religion.

Missionary Robert Park was held on Christmas Day 2009, after walking across the border to make a one-man protest about human rights violations.

Park was freed in February 2010 after the North said he expressed "sincere repentance" for his actions and had been misled by Western propaganda.

On January 25, 2010, the North detained Aijalon Mahli Gomes for crossing the border illegally and sentenced him to eight years' hard labour.

Gomes was said to be a devout Christian. When living in South Korea as an English teacher, he had joined rallies denouncing the North's human rights record.

Gomes was freed last August after former US President Jimmy Carter flew to Pyongyang to intercede.

Carter is due to visit the North again soon, reportedly late this month. Jo Sung-Rae of the Seoul-based Christian activist group Pax Koreana predicted the former US leader would also secure the release of the latest detainee.

Carter has said he would try to revive stalled six-party talks on the North's nuclear disarmament and address humanitarian woes during his visit.

United Nations food agencies say more than six million people -- a quarter of the North's population -- urgently need food aid.

After a year of high tensions in 2010, the North appears to making more conciliatory gestures to the United States and South Korea as it appeals for international food aid.

Jo told AFP that Carter's visit was initially scheduled for early March but delayed by Pyongyang to late April after Japan's quake and tsunami.

"The North apparently didn't want to share global headlines with anyone when they would make major news," he said.