North Korea is believed to be excavating a new tunnel at its Punggye-ri nuclear test site for nuclear testing, a U.S. research institute said Wednesday, citing recent satellite imagery.

The new tunnel is in a new area of the site in addition to the three others where the North has either conducted nuclear tests or excavated tunnels in the past, 38 North said.

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

A review of imagery over the past year shows significant construction in the area beginning in April, and imagery from October and November shows an additional structure and what appear to be significant tailings, indicating excavation of a new tunnel is underway, 38 North said.

North Korea has so far conducted three nuclear tests, in 2006, 2009 and 2013, all at the Punggye-ri site.

In the run-up to the 70th anniversary of the founding of the North's ruling Workers' Party on Oct. 10, fears had grown that the country could conduct its fourth nuclear test or a long-range rocket launch, but no such provocations took place.

In October, South Korea's main intelligence agency, the National Intelligence Service, reported to lawmakers that the North is preparing for its fourth nuclear test, although a test is not imminent. (Yonhap)