SEOUL (AFP) - Two North Koreans swam across the sea border with South Korea on Thursday, Seoul's Unification Ministry said, in a rare maritime defection across the tense boundary.

Two men - in their 20s and 50s - swam across the Yellow Sea border to the South's island of Gyodongdo at around 4.00 am (1900 GMT Wednesday), where they were spotted by local marines.

"The marines found them coming across the border... and both expressed a desire to defect. They are believed to be a father and a son," the spokesman said. "They are being interrogated as part of a normal process for defectors," she added.

The heavily fortified island is about 2.5 kilometres away from the closest North Korean shore. It is rare for defectors to try and swim over the border, although it was not immediately clear how far the two men had come.

Last month, a North Korean man sailed across the border in a boat. Most North Koreans who flee repression and poverty at home cross the porous frontier with China first before travelling to a South-east Asian nation and eventually arriving in the South.

Few defect across the heavily fortified land border between two Koreas, marked by barbed wires and guarded by tens of thousands of troops on both sides. Fishing vessels that stray across the border - unintentionally or otherwise - are often seized by the South Korean coastguard. The crew members are then given the choice of remaining in South Korea or being repatriated.