North Korea appears to have ramped up its nuclear weapons production over the past 18 months in defiance of strict United Nations sanctions and could possess as many as 21 atomic bombs, a U.S. research institute said Tuesday.

The Washington-based Institute for Science and International Security said in a report that Pyongyang had added 4-6 nuclear bombs from an earlier estimate of 10-16 as of the end of 2014. The surge in the number of atomic weapons, the report said, was based on “additional production of separated plutonium and weapon-grade uranium” from spent nuclear fuel at the North’s Nyongbyon nuclear complex.

It said the North now possesses “about 13-21 nuclear weapons.”

However, it underscored that there could be more than 21 weapons as a second refining plant may also be in use. In August, IHS Janes said satellite images appeared to show that the North had put into operation a second centrifuge hall at Nyongbyon.

“This estimate, despite not being comprehensive, shows that North Korea appears to be significantly increasing its nuclear weapons capabilities,” the institute’s David Albright and Serena Kelleher-Vergantini said in the report.

The institute said it had independently confirmed activities inside the main building at the Nyongbyon site’s Radiochemical Laboratory.

A U.S. government source told Reuters last week that the North had separated plutonium — which can also be used to make nuclear bombs — at the laboratory.

The report said that while the apparent activity inside it was not directly visible, “some indirect signatures associated with plutonium separation have been visible and continue to be present” in recent satellite imagery.

The report also noted a possible isotope separation facility at the site. While the exact purpose and operational status of this new facility remains unknown, it said, an analysis suggested it was suitable for separating tritium for thermonuclear weapons North Korea has vowed to develop.

North Korea mothballed the Nyongbyon reactor in 2007 under an aid-for-disarmament agreement with the U.S., but began renovating it after carrying out its third nuclear test in 2013.

The North announced its fourth atomic test on Jan. 6. Experts say it is unlikely that Pyongyang will conduct another test in the near future so soon after.

“North Korea likes to space out its tests over time to achieve maximum PR impact,” said Robert E. Kelly, a professor of international relations at Pusan National University in South Korea. “If they start testing too often, their tests won’t capture the global front page anymore.”

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