“North Korea is ready to carry out a nuclear test at any time, depending on the leadership’s decision. We are keeping a close eye on its nuclear activities,” the official said.

South Korea has been aware that the North could move ahead with another test at any time since it conducted its last nuclear test in September.

North Korea has conducted five nuclear tests and a series of missile launches, in defiance of UN sanctions, and is believed by experts and government officials to be working to develop nuclear-warhead missiles that could reach the United States.

Fox News in the United States reported on Thursday that the North was in the final stages of preparing for another nuclear test, possibly within the next few days. The network cited US officials with knowledge of recent intelligence.

It quoted one of the officials as saying as saying the test could come as early as the end of the month.

“At the sites where we watch tests, we see a level of activity that’s similar to what they’ve done before other tests,” a US defence official told AFP on condition of anonymity.

The Washington-based think tank 38 North said in February satellite imagery showed the North’s nuclear site continued low-level activity in a possible sign that it could conduct another test soon. However, it said it was unclear exactly when such a test might take place.

The South Korean military has said several times since the September test that Pyongyang was ready to conduct another nuclear blast at any time, and that a tunnel was available at the site to do so.

North Korea said last year it had mastered the ability to mount a warhead on a ballistic missile and has been ratcheting up a threat that its rivals and the United Nations appear powerless to contain.

Reports of a possible nuclear test came as the UN Security Council on Thursday blasted the North’s latest ballistic missile launch and an earlier rocket engine test as yet another breach of UN resolutions.

Both the missile launch and the engine test are “in grave violation” of Pyongyang’s international obligations under six previous resolutions passed to prevent the country from conducting such tests, a statement said.

In addition to “expressing serious concern” over the country’s “increasingly destabilising behaviour,” Pyongyang was called upon to refrain from such activities.

Under past UN resolutions, North Korea is barred from any use of ballistic missile technology. But a spate of six sanctions since its first nuclear test in 2006 have failed to dissuade the country from pursuing what it insists are defensive weapons.

Reuters, Agence France-Presse. Kyodo