A South Korean government source has said Seoul and Washington are on standby for fresh sanctions against the Democratic People's Republic of Korea after speculations that the DPRK is preparing for a fifth nuclear bomb.

South Korean Defence Ministry said it is closely monitoring the DPRK's activities, especially as it just marked the 84th birthday of its army. Last Saturday, Pyongyang conducted a submarine-launched ballistic missile test, drawing international condemnation.

The United Nations called the move "extremely troubling" and urged Pyongyang to "cease any further provocative action."