North Korea may be adding seven nuclear bombs to its arsenal annually and could have about 20 bombs by the end of 2016, according to a U.S. analyst.

Siegfried S. Hecker, a nuclear weapons expert at Stanford University who has visited North Korea’s nuclear facilities as recently as 2010, states on 38 North, a Johns Hopkins University website dedicated to North Korea, that Pyongyang could add 330 pounds of highly enriched uranium to an existing stockpile of about 660 to 880 pounds.

That amount, 330 pounds, is sufficient to produce six nuclear bombs, according to Hecker.

While North Korea’s highly enriched uranium program is growing, the country’s capacity to produce plutonium “remains limited” to 13 pounds per year.