North Korea is believed to have shut down its nuclear reactor at the Yongbyon complex, starting a process that could lead to the production of more weapons-grade plutonium, South Korean officials said on Monday.

“We are treating this matter very seriously,” Kim Sook, a senior South Korean foreign ministry official, said. “We learned of the halt to operations, and have confirmed it through various means,” Mr Kim told KBS Radio. “I think we'll have to deal with the suspension of the 5MW reactor as a weighty issue.”