North Korea said it would restart its only nuclear reactor to provide plutonium for its weapons program, further ratcheting up tensions on the Korean peninsula and drawing swift international criticism.

"Nuclear threats are not a game," said U.N. Secretary-General Ban Ki-moon at a news conference in Andorra. "The current crisis has already gone too far."

U.S. Secretary of State John Kerry called the North's recent belligerent rhetoric "unacceptable" and said the U.S. will defend itself as well as South Korea and Japan from any threat from the North.

Experts said it would take many months for North Korea to get the reactor operational again and much longer to extract enough plutonium to make weapons. The reactor at the Yongbyon plant, 55 miles north of Pyongyang, was closed in 2007, and the North Koreans destroyed the cooling tower the following summer as part of an aid-for-disarmament deal that soon collapsed.

The North's move came after rapidly deteriorating relations between Pyongyang and Seoul and after several symbolic but muscular U.S. military maneuvers on the Korean peninsula.