North Korea has tightened antivirus efforts in an eastern inter-Korean border region after warning its people about coronavirus transmissions through anti-Pyongyang leaflets sent from South Korea.

The state-run Rodong Sinmun reported on Friday that the North is stepping up efforts to prevent the spread of COVID-19 in the border county of Kosong in Kangwon Province, particularly in sea and forest areas.

The newspaper said, "We are putting all-out efforts to educate the public to strictly follow the guideline of immediately reporting any unusual occurrence to the relevant authorities," pointing to "things such as dead animals."

On Thursday, it also warned people against "strange objects" floating in the air, saying that it could carry the coronavirus.

An activist group in South Korea, consisting of North Korean defectors, said last week that it sent 500,000 leaflets over the border via large balloons despite a recently enacted ban on such actions.

In response, Kim Yo-jong, the sister of North Korean leader Kim Jong-un, lambasted Seoul for failing to stop the latest leaflet campaign, which she described as an "intolerable provocation."

North Korea has claimed to be coronavirus-free but has taken tough measures and imposed border controls since the start of the pandemic.