South Korean guards fired up to 20 warning shots at North Korean troops searching for a soldier who had defected on Thursday, Yonhap news agency said, after officials in the South confirmed the latest defection across the heavily militarised border.

Thursday's defection came about five weeks after another North Korean soldier suffered critical gunshot wounds during a dash across the border on November 13.

Two North Korean civilians found in a fishing boat had also sought to defect, officials in the South said on Thursday. There was no immediate comment from the secretive North about the latest incidents.

The defections come at a time of heightened tension on the Korean peninsula because of North Korea's missile and nuclear programmes, which it is pursuing in defiance of international pressure that includes United Nations resolutions.

In the latest North Korean military defection, a low-ranking soldier crossed the border at around 8:04 am (2304 GMT Wednesday), South Korea's Joint Chiefs of Staff spokesman Roh Jae-cheon said. No shots were fired at the soldier, Roh said.

Surveillance equipment detected the defecting soldier despite heavy fog that limited visibility to about 100 metres (110 yards), Roh later told a briefing.

"Today a North Korean soldier defected through our military’s general outpost in the centre-western region," Roh said.

"Our military secured his safety, and a questioning will be conducted by related agencies to find out the background of his defection and motive, etc," he said.

Reuters was unable to immediately confirm Yonhap's report about warning shots being fired from the South as North Korean troops searched for the defecting soldier and few other details were available.

South Korea's Unification Ministry said maritime police had found two North Korean men drifting in a small boat off the coast on Wednesday.

The pair "expressed their willingness to defect", a ministry official said. Their claim for asylum was being investigated, the official said.

The North Korean soldier who was shot several times during a daring dash across the border on Nov. 13 has since been identified as 24-year-old Oh Chong Songbeen.

Oh was transferred to a military hospital last week from a trauma centre at Ajou University Hospital south of Seoul, where his treatment for gunshot wounds and pre-existing conditions included two major operations.

Intelligence officials said they would soon begin questioning Oh.