North Korea has said it will restart its only plutonium reactor and vowed to strengthen its nuclear arsenal, in a move that further ratchets up tensions on the Korean peninsula following weeks of belligerent rhetoric and threats from young leader Kim Jong-un.

Pyongyang said Tuesday it would rebuild the Yongbyon plant, 55 miles north of the capital, that generated plutonium from spent fuel rods until it was shuttered under a 2007 disarmament agreement that soon collapsed.

The move indicated that Pyongyang will use the 5 megawatt uranium enrichment plant to produce material for nuclear weapons, reports the Financial Times.

US Secretary of State John Kerry said the US would defend itself as well as South Korea and Japan from the North’s “provocative act”, the Wall Street Journal reports.

“The bottom line is that what Kim Jong-un has been choosing to do is provocative, it is reckless and the United States will not accept the DPRK as a nuclear state,” Kerry said, following a meeting with the South Korean foreign minister.

Experts, however, said the statement represents a less immediate threat given the long time-frame needed to get the plutonium reactor running.

Evidence of North Korea’s more imminent threats came Wednesday when it blocked the South Korean entry into a joint industrial zone, cutting off a key strand of inter-Korean cooperation.