North Korea has the internal ability to produce tritium, a basic element for making hydrogen bombs, although the country has yet to weaponize it, South Korean news agency Yonhap quoted a US nuclear expert as saying Tuesday.

“The evidence is quite clear that North Korea is able to produce tritium, which is necessary for a hydrogen bomb to create fusion. So you need tritium when you are going to have hydrogen bombs,” Siegfried Hecker, a Stanford University professor told a group of journalists in Seoul. Hecker was allowed to take a look at Pyongyang’s uranium-enrichment facility in Yongbyon during a 2010 trip to North Korea.

North Korea claimed it has tested a hydrogen device in January last year, though this has never been proven.