North Korea may be preparing to test out its thermonuclear weapons at the Punggye-ri nuclear test site which is located in Kilju County in the North Hamgyong Province, a South Korean military unit said Sunday.

South Korea’s Chemical, Biological and Radiological (CBR) Defense Command said that North Korea has laid the groundwork to develop thermonuclear weapons and may already be producing tritium, a radioactive isotope necessary to build more sophisticated nuclear weapons, Seoul-based Yonhap news agency reported.

"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," CBR said, according to Yonhap.

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

Countries, working to produce nuclear weapons, usually first develop weapons that use nuclear fission to break larger atoms, such as Uranium and Plutonium, into smaller atoms. Thermonuclear weapons or fusion weapons, which combine small atoms like Hydrogen, are more complicated to make. It takes countries many years to independently develop the technology needed to create a fission bomb and then make the subsequent jump to fusion weapons.

However, as part of making fusion weapons, some countries develop “boosted” fission bombs which use a small amount of fusion to boost the fission process, releasing more energy.

"The North could detonate its boosted fission weapon, but we don't believe it is yet capable of directly testing hydrogen bombs," the CBR Defense Command said

"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," according to the CBR report.

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