North Korea may have doubled its nuclear stockpile in the last 18 months, according to the latest research.

The Washington-based Institute for Science and International Security (ISIS) said the hermit state could have made at least six more weapons since the end of 2014.

At that time, ISIS estimated Kim Jong Un had 10 to 16 nuclear weapons based on analysis of the country's production of weapons-grade uranium and plutonium recovered from spent nuclear fuel.

In revised estimates contained in a report provided to Reuters, the institute's David Albright and Serena Kelleher-Vergantini said North Korea may have added another four to six weapons since then, for a total of 13 to 21 or even more today.

South Koreans watch a news report on North Korea's first apparent hydrogen bomb test in Seoul in January. A U.S. research institute believes North Korea may have doubled its nuclear stockpile in the last 18 months

The report said the 13 to 21 estimate did not take into account the possible production of additional highly enriched uranium at a second centrifuge plant thought to exist in North Korea.

'Nonetheless, this exercise, despite not being comprehensive, shows that North Korea could be significantly increasing its nuclear weapons capabilities,' the report said, adding that most of the increase could be attributed to the production of weapons-grade uranium.

The report came a week after a senior U.S. State Department official said North Korea had restarted production of plutonium fuel, indicating that it planned to pursue its nuclear weapons program in defiance of international sanctions that followed its fourth nuclear test in January.

Plutonium also can be used to make nuclear weapons.

The institute's report said the group independently confirmed activity inside the radiochemical laboratory at North Korea's Yongbyon nuclear site.

The institute's report said the group independently confirmed activity inside the radiochemical laboratory at North Korea's Yongbyon nuclear site

It said commercial satellite imagery from June 8 did not show direct signs, but noted 'indirect signatures associated with plutonium-separation' there.

These included the removal of tanks previously spotted in front of the laboratory's reception building and signs that a coal-fired steam generation plant may have been active on June 8.

The report also noted signs of external activity at a site the institute identified as a possible isotope-separation facility that could be used to produce tritium for thermonuclear weapons that North Korea has said it intends to develop.

North Korea vowed in 2013 to restart all its nuclear facilities, including the main reactor and a smaller plant at Yongbyon, which was shut down in 2007 as part of an international disarmament-for-aid deal that later collapsed.