North Korean authorities have imposed a lockdown on the border city of Kaesong after discovering what they called the country's first suspected case of the novel coronavirus, state media reported Sunday.

Leader Kim Jong Un convened an emergency politburo meeting on Saturday to implement a "maximum emergency system and issue a top-class alert" to contain the virus, official news agency KCNA said.

If confirmed, it would be the first officially recognised COVID-19 case in the North where medical infrastructure is seen as woefully inadequate for dealing with any epidemic.

KCNA said a defector who had left for the South three years ago returned on July 19 after "illegally crossing" the heavily fortified border dividing the countries.

But there have been no reports in the South of anyone leaving through what is one of the world's most secure borders, replete with minefields and guard posts.

Pyongyang has previously insisted not a single case of the coronavirus had been seen in the North despite the illness having swept the globe, and the country's borders remain closed.

The patient was found in Kaesong City, which borders the South, and "was put under strict quarantine", as would anybody who had come in close contact, state media said.

It was a "dangerous situation... that may lead to a deadly and destructive disaster", the media outlet added.

Kim was quoted as saying "the vicious virus could be said to have entered the country", and officials on Friday took the "preemptive measure of totally blocking Kaesong City".

The nuclear-armed North closed its borders in late January as the virus spread in neighbouring China and imposed tough restrictions that put thousands of its people into isolation, but analysts say the North is unlikely to have avoided the contagion.

South Korea is currently recording around 40 to 60 cases a day.

Earlier this month Kim warned against any "hasty" relaxation of anti-coronavirus measures, indicating the country will keep its borders closed for the foreseeable future.