A group of North Koreans - including children - has been found adrift in a boat in the Yellow Sea off the coast of South Korea.

Members of the group say they want to defect.

The 21 North Koreans were found in a five-tonne wooden boat by sailors on a South Korean Navy vessel in the Yellow Sea on Sunday, the Joint Chiefs of Staff said in a press statement. Earlier reports said they were found Tuesday.

They were then taken by patrol boat to the port of Incheon, where the group, which includes nine children, expressed a desire to defect.

"They have been under interrogation by relevant authorities," the military statement said.

It was not immediately clear why the navy and coastguard withheld the news from the public for six days.

Saturday's announcement represented the fourth occasion this year that North Korean defectors have reached South Korea by crossing the Yellow Sea frontier.

Hundreds of North Koreans flee the impoverished state every year, most escaping over the land border into China.

China normally returns fugitives from the North even though they could face harsh punishment in their homeland for escaping. The policy is denounced by rights groups, who they should get refugee status.

Seoul's policy is to accept all North Koreans who wish to stay in the South, while repatriating those who stray across the sea border by accident.

In February, a boatload of 31 North Koreans arrived in South Korea, sparking weeks of acrimony. That boat drifted across the Yellow Sea border in thick fog, possibly unintentionally.

Seoul returned 27 of the 31 people on board but refused to hand over the other four, saying they had freely chosen to stay in the South.

Nine North Korean refugees, including three children, were picked up by Japan's coastguard in September after leaving the North's east coast. They arrived in South Korea last month.

Two North Koreans were also admitted to the South last month after they were found adrift in a small boat off its east coast.