North Korea on Monday released and deported a South Korean man studying at New York University who was arrested back in April for illegal entry, officials said.

Joo Won-Moon, a 21-year-old with permanent US residency, was repatriated through the border truce village of Panmunjom on the inter-Korean border at around 5:30 pm (0830 GMT), Seoul's Unification Ministry said.

Soon after Joo walked over the border in Panmunjom, he was held by security officials for questioning about his unauthorised trip to North Korea, a ministry official told AFP.

The move is likely to be seen as a goodwill gesture ahead of a key political anniversary, but while Seoul welcomed Joo's release, it also urged Pyongyang to repatriate three other South Korean citizens being held in the North.

Joo's release came after he was presented to the media in Pyongyang on September 25 and read out what appeared to be an officially approved statement, admitting his guilt and singing the country's praises.

Unlike some other foreign citizens detained in the North, he was never put on trial.

"We welcome the decision to repatriate our citizen... and we urge North Korea to release three other (South Korean) people being detained," the ministry said earlier in a statement.

Two South Koreans were sentenced to life with hard labour in June on espionage and other charges. A South Korean missionary was given a similar sentence in May, 2014 -- also on espionage charges.

Joo was arrested after crossing the Yalu River into the North from the Chinese border city of Dandong on April 22.

He told CNN in an interview in May that he had crossed two barbed-wire fences and walked through farmland until he reached a large river. He followed the river until soldiers arrested him.

"I thought that by my entrance .... some great event could happen and hopefully that event could have a good effect on the relations between the North and (South Korea)," he said at the time.

Joo, who has permanent US residency, was born in Seoul, moved to Wisconsin with his family in 2001 and then moved again to Rhode Island.

Numerous foreigners, mainly Americans and often evangelical Christians, have crossed illegally into the North over the years.

The North normally releases them after they have served a short prison term, sometimes in response to a visit by a senior US official.

Joo's release comes as North and South Korea are preparing to hold a rare reunion later this month for families separated by the 1950-53 Korean War.

There are concerns that the event could be cancelled, amid speculation that the North is preparing a long-range rocket test to mark the 70th anniversary on October 10th of its ruling Workers' Party.

Any such launch would violate UN resolutions and trigger a spike in military tensions on the Korean peninsula.