Defence Ministry spokesman Moon Sang-kyun said that according to intelligence authorities of South Korea and the US, North Korea was "always ready" to carry out additional nuclear test in its main Punggye-ri nuclear test site, Xinhua news agency reported.
His comments followed North Korea successfully staging an explosion test of a nuclear warhead that can be mounted on ballistic missiles "at will".
The warning of additional test is based on three tunnels excavated at Punggye-ri near which all of its underground nuclear tests were conducted.
"If North Korea conducts additional nuclear test, it would be possible to happen both from a branch of the second tunnel or the third tunnel where all preparations are already done," said Moon.
All of Pyongyang's nuclear device tests took place in Punggye-ri. The first test was conducted in the first tunnel, while the second, the third and the fourth detonations came from the second tunnel.
The site of the fifth nuclear test is some 500 metres away from where the fourth test was conducted, according to Yonhap news agency report citing government sources.
Pyongyang reportedly completed all preparations for a nuclear test at the third tunnel, where any atomic device test has not occurred yet, the sources added.