North Korea has freed a South Korean national who is a student at New York University, in a possible sign it wants better ties with rival Seoul and may back away from a recent threat to launch a long-range rocket later this month.

North Korean state media said it "deported" Won Moon Joo at the border village of Panmunjom as a "humanitarian" measure about six months after the 21-year-old had been arrested for crossing the Chinese border into North Korea.

South Korean officials confirmed Mr Joo's repatriation. The National Intelligence Service, South Korea's main spy agency, said it will investigate whether he violated the country's anti-North Korean security law, which prohibits unapproved travel to the North.

The exact motivation for his travel to North Korea was not clear.