SEOUL: A young North Korean man has defected to South Korea by crossing the tense and heavily fortified land border, a government official said Wednesday.

He is being interrogated by South Korean security authorities, the official said on condition of anonymity, without disclosing details

The man in his early 20s was spotted by South Korean soldiers at around 5 pm on Tuesday in Cheorwon, 120 kilometres (72 miles) northeast of Seoul, Yonhap news agency said.

Defections across the land frontier, which is guarded by barbed wire and minefields in many places, are rare.

Almost all North Korean refugees go first to China and travel on to third countries before winning resettlement in the South.

Two weeks ago the South's navy detained a small boat with 11 men and 20 women aboard, just south of the disputed Yellow Sea maritime border. Officials said they might have drifted into South Korea.

The North has called for their repatriation. The South normally returns North Koreans who drift across the border by accident and who express a wish to return.

The group's motives were still under investigation Wednesday.

More than 20,000 North Koreans have arrived in the South since the end of the 1950-53 war.