North Korea again called Monday for stepped-up efforts to prevent the spread of COVID-19 amid growing cases of fever patients there, while the secretive nation has claimed to be coronavirus free.

South Korea's spy agency said earlier that a temporary lockdown imposed in the North last Wednesday is suspected to have been related to COVID-19, though it also raised the possibility of an outbreak of waterborne diseases, such as typhoid.

The Rodong Sinmun, an organ of the ruling Workers' Party, urged people not to let their guard down and stressed that even a single violation of antivirus measures can bring down the whole quarantine system the country has built.

"Our most dangerous enemy right now, at a time when emergency antivirus efforts have protracted, is inattentiveness and laxity," the paper said.

The North has implemented relatively swift measures to ward off the pandemic since January 2020, including border controls of people and goods between the North and China.

Last month, freight train operations on a route between the Chinese city of Dandong and the North Korean border city of Sinuiju were apparently suspended amid the spread of the virus in China, after it resumed operations in January in about 1 1/2 years. (Yonhap)