WASHINGTON (Yonhap) — North Korea is estimated to be holding up to 22 nuclear weapons worth of fissile material, a U.S. research institute said Wednesday.

The Institute for Science and International Security made the estimate in a report, saying that as of last year the communist nation was believed to be holding up to 34 kilograms of plutonium and 240 kilograms of weapons-grade uranium if Pyongyang has two uranium enrichment plants.

But the estimate could further increase because Pyongyang could extract plutonium from spent nuclear fuel used in the 5-megawatt nuclear reactor that it restarted in 2013, the institute said.

As part of six-party denuclearization talks, North Korea made a declaration of its past nuclear activity in 2008. Citing an unidentified source, ISIS said Pyongyang is believed to have reported to the U.S. at the time that it extracted a total of 37-38 kilograms of plutonium.

In addition, the North is believed to have extracted about eight kilograms of plutonium from spent fuel it was holding until it froze its nuclear facilities under a six-party deal in 2007.

Of the total, the North is estimated to have used up to 12 kilograms of plutonium in its three nuclear tests in 2006, 2009 and 2013, leaving the total stockpile at the end of last year at between 30-34 kilograms, the institute said.

In 2010, the North also revealed that it was running a uranium enrichment facility, adding to international concerns about its nuclear capabilities. Uranium, if highly enriched, can be used to make weapons, providing Pyongyang with a second way of building atomic bombs, in addition to its existing plutonium-based program.

The North is believed to have produced about 100 kilograms of weapons-grade uranium from the facility. But in case the North has another secret uranium enrichment facility, the total amount of weapons-grade uranium would be about 240 kilograms, ISIS said.

In addition to the three nuclear tests, the North has also conducted a series of long-range missile or rocket launches since 1998. In its most recent launch in late 2012, the North succeeded in putting a satellite into orbit.

Analysts have warned that it is only a matter of time until the North develops nuclear-tipped missiles. Some experts have recently warned that the communist nation’s nuclear arsenal could expand to as many as 100 bombs by 2020.