The man's departure only came to light when Pyongyang -- which insists it has not had any coronavirus cases -- announced at the weekend that a "runaway" who had returned across the heavily fortified Demilitarized Zone was suspected of having the disease.

Defections from the South to the North are extremely rare, and doubly so across the DMZ, which divides the peninsula and is one of the world's most secure borders.

But the South Korean military later confirmed a 24-year-old man surnamed Kim had gone North by crawling through a drainage channel on Ganghwa island, northwest of Seoul, and then swimming across the Han river.

On Friday, the joint chiefs of staff (JCS) said that Kim was detected on his journey seven times by surveillance cameras or thermal observation devices, but the military did not intervene.

A guard at a post about 200 metres from the water conduit spotted "lights" when Kim arrived in a taxi at 2:18 am, but took no action, they added.

Marine corps general Baek Kyung-soon had been removed from his position for the security failure, Seoul's defence ministry official told AFP.

The JCS said it took Kim about 12 minutes to crawl through the drain, and around an hour to swim across the Han to his destination.

Kim originally defected to the South in 2017, also by swimming across the river. He was being investigated on rape allegations in the South before his return.

Rights groups say defectors face severe punishment if they return to the North.

The South's health authorities said his name did not appear in the database of confirmed coronavirus cases, nor lists of their contacts.

But analysts say the North is seizing on his arrival to point the finger at Seoul over the coronavirus, after months of denying it had any cases.

Inter-Korean relations have been in a deep freeze following the collapse of a summit in Hanoi between Kim and US President Donald Trump early last year over what the nuclear-armed North would be willing to give up in exchange for a loosening of sanctions.

Friday's announcement comes days after the South's defence minister Jeong Kyeong-doo apologised over Kim's departure, saying: "I have no excuse even if I'm told off a hundred times over this."