SEOUL (Reuters) - North Korea on Saturday said it had arrested a South Korean college student, holding a US green card, for illegal entry into the country.

Joo Won Moon, 21, from New York, was caught on April 22 crossing from the Chinese side of the Yalu River, according to North Korea's KCNA news agency. Joo admitted violating North Korean law, state media said.

North Korea's KCNA news agency reported that Joo was a student at New York University (NYU).

The arrest comes after North Korea said in late March that it had detained two South Koreans, accusing them of spying after they crossed over from the Chinese border city of Dandong.

Dandong is home to many ethnic Korean Chinese traders who deal with both North and South Korean businessmen. It is also home to South Korean and western Christian missionaries trying to operate in North Korea.

A number of US missionaries have been arrested in the past, with some allowed to return home after interventions by high-profile US figures.

But, despite pleas from Seoul to release, North Korea has been holding South Koreans including a South Korean missionary who was sentenced to life with hard labour last year for espionage and setting up an underground church.

The two Koreas remain technically at war, as their 1950-53 civil conflict ended in a truce rather than a peace treaty.