A former International Atomic Energy Agency (IAEA) official cast doubts on a recent report estimating that North Korea’s stockpile of nuclear weapons could reach 100, potentially by 2020.

“It’s not a normal forecast, even in a worst-case scenario, that (North Korea) could build 100 nuclear weapons in six years,” former IAEA deputy director general Olli Heinonen was quoted as saying in an interview with Voice of America on Tuesday, referring to the U.S.-Korea Institute report North Korea’s Nuclear Futures: Technology and Strategy, authored by Joel Wit and Sun Yong Ahn.

“Building a nuclear weapon is a very complicated process,”