North Korean soldiers shot dead a suspected South Korean defector at sea and burned his body as a coronavirus precaution after he was interrogated in the water over several hours, Seoul military officials said Thursday.

It is the first killing of a Southern citizen by North Korean forces for a decade, and comes with Pyongyang at high alert over the pandemic and inter-Korean relations at a standstill.

The fisheries official disappeared from a patrol vessel near the western border island of Yeonpyeong on Monday, a South Korean military official told AFP.

More than 24 hours later, North Korean forces located him in their waters and questioned him from a patrol boat, he said, with his interrogator wearing protective equipment.

He was killed around six hours after being found, according to the official.

"He was shot dead in the water," he said. "North Korean soldiers poured oil over his body and burnt it in the water.

"We assess it was carried out under the North's anti-coronavirus measure," he added.

According to the defence ministry, the people who burnt the body wore gas masks and protective clothing.

There was no immediate comment from Pyongyang on the incident and it was not possible to independently verify the South Korean military's account.

Pyongyang has closed its borders and declared an emergency to try to protect itself against the virus which first emerged in neighbouring China.

The man was wearing a lifejacket and his shoes had been found on board the South Korean boat, the official said -- indicators suggesting he entered the water voluntarily.

"We have obtained intelligence that he had expressed his intention to defect while being questioned," he added.

He declined to go into detail about the source of the information.

But the South Korean military is known to intercept radio communications by the North's forces.

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 a statement.

South Korean media reports said the man was in his forties with two children, but had recently divorced and had financial problems.

- 'Shoot to kill' -

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.

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.

That same month, North Korean officials put the border city of Kaesong under lockdown after a defector who had fled to the South three years ago sneaked back over the heavily fortified border, 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.

This week's incident is the first time in 10 years that North Korean forces have killed a Southerner.

In November 2010, Pyongyang's military bombarded Yeonpyeong island -- close to this week's incident -- killing two civilians and two marines.

It came months after a torpedo fired from a North Korean submarine sank South Korea's Cheonan warship, killing 46 seamen, although Pyongyang denies responsibility.

In 2008, a North Korean soldier gunned down a female South Korean tourist who walked into a forbidden area at the North's Mount Kumgang resort, prompting Seoul to suspend the money-spinning visits.