North Korea is believed to have produced an additional four to six nuclear weapons since late 2014, with its total arsenal now estimated at between 13-21 weapons, a U.S. think tank said Tuesday.

The Institute for Science and International Security released the assessment in a report, making estimates about the amounts of weapons-grade plutonium and highly enriched uranium the North is believed to have produced at its Yongbyon nuclear complex.

The assessment came after recent reports that the North has started reprocessing spent nuclear fuel to harvest plutonium for nuclear weapons.

ISIS had estimated the North's nuclear arsenal at 10-16 weapons as of late 2014.

"Since the end of 2014, or during the last 18 months, North Korea has added an estimated 4-6 nuclear weapons to this value, based on additional production of separated plutonium and weapon-grade uranium at Yongbyon, where any contribution of a second centrifuge plant is ignored," ISIS said.

"Thus, as of June 2016, North Korea has about 13-21 nuclear weapons, where one weapon was subtracted to reflect the underground test in early 2016," it said.

The upper bound, or 21 weapons, could be greater because it does not include the effect of any weapons-grade uranium produced in a possible second centrifuge plant.

"Nonetheless, this estimate, despite not being comprehensive, shows that North Korea appears to be significantly increasing its nuclear weapons capabilities," the report said. (Yonhap)