North Korean leader Kim Jong-un / Yonhap

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.