North Korea's official newspaper on Thursday urged people to stay alert against coronavirus infections through "strange objects" floating in the air, days after the regime issued a harshly worded statement against propaganda leaflets sent from South Korea.

Last week, an activist group consisting of North Korean defectors in South Korea said that it released around 500,000 leaflets into the North via large balloons despite a recently enacted ban on such cross-border leafleting.

The sister of North Korean leader Kim Jong-un issued a statement Sunday, slamming Seoul for failing to stop the leaflets, calling the launches an "intolerable provocation" and warning of unspecified "corresponding action."

On Thursday, the North's Rodong Sinmun newspaper said strange objects in the air could carry the coronavirus.

"Even when we come across a strange object flying in the wind, we must consider them as a possible route of transmission of the malicious virus rather than a natural phenomenon," the paper said, urging people to "think and move" according to the national COVID-19 guidelines.

The paper also urged people to tighten antivirus efforts, stressing that vaccines have been ineffective in preventing the spread of coronavirus variants in other countries.

North Korea was initially expected to be supplied with around 1.7 million coronavirus vaccines by May through the COVAX Facility program, but the delivery appears to have been delayed.

North Korea has claimed to be coronavirus-free but has taken relatively swift and tough measures against the pandemic, such as imposing strict border controls since early last year. (Yonhap)