A US think tank revealed late on Wednesday that recent satellite images suggest that North Korea is building a new nuclear test tunnel. The statement by the US-Korea Institute at Johns Hopkins University confirmed earlier reports by South Korean media that Pyongyang was building a fourth tunnel at its principal nuclear testing site.

"While there are no indications that a nuclear test is imminent, the new tunnel adds to North Korea's ability to conduct additional detonations over the coming years if it chooses to do so," said the website 38 North, the institute's North Korea monitoring website.

The tunnel is being constructed in a new area of the Punggye-ri site, in the northeast of the country, where three nuclear tests have previously been conducted in 2006, 2009, and 2013.

In September, North Korea announced that it was restarting its Yongboyn nuclear reactor, which was originally shut down in 2007 as part of an aid-for-disarmament deal with South Korea, the United States, Russia, China, and Japan.

Yongbyon, when running at full capacity, is able to produce about six kilograms (13 pounds) of weapons-grade plutonium a year, enough for a single nuclear bomb, according to experts.

North Korea has long claimed that its nuclear program is aimed at producing a rocket to launch a satellite into orbit, and vowed to continue its testing despite condemnation and international security concerns.

es/sms (AFP, Reuters)