SEOUL - North Korean forces shot dead a South Korean fisheries official who disappeared off a patrol vessel and ended up in Pyongyang's waters, Seoul's defense ministry said Thursday, calling it an "outrageous act".

The 47-year-old man had been on board a vessel near the western border island of Yeonpyeong, the ministry said in a statement.

After analysing intelligence, the South Korean military had "confirmed that the North fired at a South Korean national found in the northern seas and cremated his body", it said.

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

It was not immediately clear how the man came to be in the water. Earlier reports said that his shoes were found on board the patrol vessel, leading to speculation he may have been trying to defect.

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 North Korean authorities issued shoot-to-kill orders to prevent the coronavirus entering the country from China, creating a "buffer zone" in the border with special forces soldiers ready to kill.

• North Korea issues shoot-to-kill orders to prevent virus: US

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.

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© Agence France-Presse