North Korea
North KoreaReuters

UPDATE 12:45 p.m. IST -- North Korea reportedly fired several short-range missiles that covered 200 kilometres before falling into the sea, according to South Korea's Yonhap News Agency.

"Our military is keeping close tabs on the situation and standing by with a heightened defense posture," South Korea's Joint Chiefs of Staff said. 

UPDATE 12:20 p.m. IST -- North Korea launched a missile into the Sea of Japan Monday, South Korea's Yonhap New Agency reported. 

North Korea had fired a ballistic missile Friday that covered a distance of 800 kilometres before falling into the Sea of Japan.

Original Story:

North Korea has the capability to carry out its fifth nuclear test at any time, South Korea warned Monday. Pyongyang had carried out a nuclear test in January, following which it was slapped with its heaviest set of sanctions by the United Nations. A U.S.-based monitoring website last week reported "continued activity" at North Korea's Punggye-ri nuclear test site. 

Last week, North Korean leader Kim Jong Un had threatened to test a nuclear warhead and ballistic missiles capable of carrying nuclear warheads, in defiance of U.N. resolutions. The 38 North, a U.S.-based website that monitors North Korea, had said satellite imagery had shown continued activity at North Korea's Punggye-ri facility, which it said "appears ready to support new nuclear tests."

"We believe that (the North) is able to carry out a fifth nuclear test immediately," Seoul's Unification Ministry spokesperson Jeong Joon-hee said, according to Yonhap News Agency. 

South Korea said it was prepared for a fifth nuclear test by the North. "The government is preparing for all possibilities," Jeong said, according to AFP

The U.S. website had said Friday the North Korean nuclear site was "capable of supporting additional tests at any time."

"Commercial satellite imagery from March shows that the North Portal, the location of the January 2016 nuclear test, remains active with footpaths around the buildings and what appears to be water draining from the entrance along the drainage ditch," the portal said. "This may support the hypothesis that branching tunnels lead to other test chambers since the shaft used for the January test should have been completely sealed to prevent radioactivity leakage".

North Korea had conducted a nuclear test Jan. 6, with an artificial earthquake detected near the country. While the Kim Jong Un regime had claimed the country had successfully tested a hydrogen bomb, experts had cast aspersions on the claim. The isolated country also carried out a rocket launch in February, which it claimed was to put a satellite in space but which was seen as a trial of a ballistic missile launch. 

North Korea has turned up the volume of its aggressive pitch in recent weeks since the United States and South Korea began their joint military drill, with Kim Jong Un even threatening pre-emptive nuclear strikes.