North Korea said it had "ratified" a merciless attack against the United States, potentially involving a "diversified nuclear strike".
"We formally inform the White House and Pentagon that the ever-escalating US hostile policy toward the DPRK (North Korea) and its reckless nuclear threat will be smashed by the strong will of all the united service personnel and people," a spokesman for the general staff of the Korean People's Army said in a statement.
"And cutting-edge smaller, lighter and diversified nuclear strike means of the DPRK and that the merciless operation of its revolutionary armed forces in this regard has been examined and ratified," it continued.
The announcement came after the Pentagon said it was to deploy the US advanced ballistic missile defence system in the Pacific as a direct response to threats from North Korea.
The Terminal High Altitude Area Defence System will be stationed on the island of Guam within weeks while US authorities claim the deployment will "strengthen our regional defence posture against the North Korean regional ballistic missile threat". North Korea barred South Korean workers from entering the Kaesong joint industrial zone yesterday, north of their demilitarised border. The site is home to more than 100 companies, and employs 50,000 North Korean workers.
Its closure came 24 hours after the North announced it was restarting the moth-balled plutonium reactor at its Yongbyon facility, signalling its intention to step up its nuclear weapons programme. That, in turn, followed a declaration by Pyongyang that a "state of war" existed on the peninsul.
The interruption at Kaesong is seen by some experts as an ominous development, given that -- with the exception of three days in 2009 -- the complex had remained open through the string of previous crises since its launch in 2004.
The closure "is something that was less expected (and is) less directly in North Korea's interests", Patrick Cronin, a senior analyst with the Centre for a New American Security in Washington, said. "Is this a short-term demonstration of dissatisfaction with US-South Korean policy, or a portent of something more drastic?"
That "something" could be a decision to take hostage hundreds of South Korean workers still at Kaesong.
As of last night, that did not appear to be happening. According to Seoul, 33 workers returned south yesterday.