North Korea says it has arrested a South Korean student, who attends New York University and has permanent US residency, for illegally entering the country from China last month.

The North's official KCNA news agency on Satuday identified the man as 21-year-old Won Moon-Joo.

Won was arrested after crossing the Yalu River from the Chinese border city of Dandong on April 22, KCNA said.

The news agency said Won had admitted his illegal entry was a "serious violation" and added that his case was under investigation.

It offered no details as to what had motivated Won to cross into North Korea.

In March, North Korea said it had arrested two South Koreans on espionage charges, describing them as "heinous terrorists" operating out of a base in Dandong.

Many Christian groups, mostly run by South Koreans, are active along the Chinese border with North Korea.

Some conduct clandestine missionary work in the North, while others focus on helping North Koreans escape the country.