North Korean soldiers shot dead a suspected South Korean defector after interrogating him at sea, then poured oil over his body and burned it over coronavirus fears, Seoul military officials said Thursday.

The man had disappeared off a patrol vessel near the western border island of Yeonpyeong, the South Korean defence ministry said in a statement.

He was wearing a lifejacket, a military official told AFP, adding that "circumstances tell us that there was an intent to defect", without providing evidence.

According to the South's Yonhap news agency, the man was located by North Korean forces and questioned from a boat by an official wearing protective equipment.

"He was shot dead in the water," the military official told AFP. "North Korean soldiers poured oil over his body and burnt it in the water."

The burning appears to have been intended as a precaution against the coronavirus, in response to which Pyongyang has closed its borders and declared an emergency.

"We assess it was carried out under the North's anti-coronavirus measure," the military official told AFP.

The killing took place after an "order from superior authority", Yonhap cited South Korean officials as saying.

Seoul's defence ministry condemned the shooting as an "outrageous act".

"We sternly warn North Korea that all responsibilities for this incident lie with it," it said.

In July, a North Korean defector who had fled to the South three years ago sneaked back over the heavily fortified border into the impoverished nation.

His crossing prompted North Korean officials to put the border city of Kaesong under lockdown amid fears that he may have carried the coronavirus.

US Forces Korea commander Robert Abrams said earlier this month that North Korean authorities had issued shoot-to-kill orders to prevent the coronavirus entering the country from China, creating a "buffer zone" at the border. 

The isolated North -- whose crumbling health system would struggle to cope with a major virus outbreak -- has not confirmed a single case of the disease that has swept the world since first emerging in China, the North's key ally. 

Pyongyang closed its border with China in January to try to prevent contamination, and in July state media said it had raised its state of emergency to the maximum level.