South Korea is on high alert for a possible nuclear test by North Korea that could take place as early as Monday to mark the 84th anniversary of the founding of the Korean People's Army, the defense ministry said.

South Korea has been bracing for North Korea's fifth nuclear test in the runup to major national events in the communist country, including the KPA anniversary and the all-important party congress expected to take place in early May.

"Our military is maintaining thorough preparations against the possibility of North Korea carrying out a nuclear test," ministry spokesman Moon Sang-gyun said in a press briefing.

"We are keeping particularly close attention to the possibility because today marks the founding anniversary of the North's armed forces," he said.  

"As far as I know there have been signs of nuclear test preparations in connection with the national event," he also noted.

North Korea may conduct a nuclear test at a unexpected time because the country is technically ready to conduct a test at any time, the official pointed out. (Yonhap)