The Joint Chiefs of Staff (JCS) said a maritime patrol aircraft had detected an unidentified vessel in waters off South Korea's east coast on Sunday and dispatched a patrol boat.

As the boat approached, the crew on the vessel flagged for help, and the South Korean sailors confirmed that it was a North Korean ship, the JCS said in a statement.

The crew on the North Korean vessel said they had been stranded for 10 days and expressed a wish to return, it added.

They asked for food and drinking water, which was provided by the South Korean side on "humanitarian grounds", according to the statement.

"The North is also being notified through the UN command and the international marine communication network so that it can rescue the North Korean ship in distress," the statement said.

On Tuesday, a group of North Koreans crossed into South Korean waters in a wooden boat, in what appeared to be a rare defection across the maritime border.

The four North Koreans on that boat had "expressed their intent to defect", South Korea's Yonhap News Agency reported, citing an unnamed government source.