The statement concerns the nuclear reactor in Yongbyon.
"In our periodic assessments, we have been able to confirm that there is a rapid increase in activity," Grossi said at a press conference in Seoul.
"All of this points to a very serious increase in North Korea's nuclear weapons production capacity, which is estimated at a few dozen warheads."
South Korea's intelligence service estimates that isolated North Korea is operating several uranium-enrichment facilities, which is a crucial step in manufacturing nuclear warheads.
North Korea has announced that it will never give up its nuclear weapons.
