On Sunday, South Korea’s Yonhap news agency quoted an anonymous intelligence source as suggesting that Pyongyang is “clandestinely preparing a nuclear test” in the country’s northeast.

South Korea and its allies accuse North Korea of defying a UN resolution by going ahead with its rocket launch.

Recent satellite images suggest tunnels are being dug underground in Punggye-ri, Kilju County nuclear test site, the report said.

On Friday, North Korea released a statement warning against any attempts to jeopardize its plans to launch a rocket-powered satellite scheduled for later this month.

The Committee for the Peaceful Reunification of Korea in Pyongyang added that whoever “intercepts the satellite or collects its debris will meet immediate, resolute and merciless punishment”.

Pyongyang had previously performed two nuclear tests in the area between 2006 and 2009.

North Korea says it plans to launch a rocket-powered satellite named Unha-3, somewhere between April 12 and 16. The rocket is said to aim at carrying a satellite into orbit.

Pyongyang insists the launch is for peaceful and scientific purposes.

The United States, South Korea, and Japan, however, claim that it will be a camouflaged test for a long-range missile.