North Korea may be preparing to test its thermonuclear weapons capabilities at its Punggye-ri nuclear test site, a South Korean military unit responsible for nuclear defense said Sunday.

In its report, the South's Chemical, Biological and Radiological (CBR) Defense Command said the North has already laid the groundwork to develop thermonuclear weapons and may already be producing tritium, a radioactive isotope necessary to build more sophisticated nuclear weapons.

"We can't discount the possibility that the North's excavation of a new tunnel at its Punggye-ri test site could be designed for thermonuclear weapons tests," said the command, under the direct control of the Defense Ministry. "Considering its research of nuclear technology, its history of underground and projectile tests, and elapsed time since its nuclear development, North Korea has the foundation for thermonuclear weapons."

North Korea has so far conducted three nuclear tests, in 2006,

2009 and 2013, all at Punggye-ri. Last week, the U.S. research institute 38 North said, based on its new high-resolution commercial satellite imagery, North Korea is forging ahead with its excavation of the new tunnel at Punggye-ri.

In October, Seoul's National Intelligence Service reported to lawmakers that Pyongyang was preparing for its fourth nuclear test, although a test is not imminent.

The CBR Defense Command said should North Korea carry out its fourth nuclear test, it will take a new format.

"The North could detonate its boosted fission weapon, but we don't believe it is yet capable of directly testing hydrogen bombs," it added.

In December, North Korean leader Kim Jong-un stated that his country had developed hydrogen bombs, though the claim was met with skepticism by experts.

The CBR Defense Command added the North may also be building a light water reactor to produce tritium.

"Technologically advanced nations have tested and developed boosted fission weapons about 10 years after their initial nuclear tests," the command said. "North Korea, 15 years after developing its nuclear capability, may be going ahead with heavy hydrogen production and warhead design. The North, however, doesn't appear to be in the final stages of completing its hydrogen bombs." (Yonhap)