The South Korean navy has rescued a group of North Koreans after two fishing boats were found sinking near the disputed Yellow Sea border, a military spokesman said Friday.

The rescue on Thursday came a day after the South reacted to alleged shelling at the border flashpoint by firing warning shots.

South Korean authorities were questioning four North Koreans after they were plucked from two boats off Baengnyeong island near the border, the Joint Chiefs of Staff spokesman said.

Another three, who were found adrift in a third fishing boat near the same island after running out of fuel, were given more fuel and allowed to return home early on Friday, the spokesman said.

"We've confirmed the three had no intention to defect to the South. Barring intentional transgression, we usually return fishing boats to the North," he said.

Seoul said its marines fired warning shots on Wednesday after its communist neighbour twice lobbed shells near the disputed maritime border.

The North's military denied the shelling, claiming the sounds came from "normal blasting" from construction work.

The South responded by saying the shells were clearly seen from a frontline observation post landing near the border.

The area has seen bloody naval skirmishes in recent years, with a North Korean artillery attack last November killing four people.