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."