The United States said on Wednesday that North Korea's nuclear program and the increased range and precision of its missiles were of great concern, a day after the isolated Asian country said it had the power to deter a US nuclear threat with a pre-emptive strike if necessary.
The US mission to International Organizations in Vienna, home to the UN's International Atomic Energy Organisation, said the restart of North Korea's graphite-moderated nuclear reactor could enable it to produce additional plutonium for weapons in the near future.
"These activities are clear violations of multiple (UN) resolutions and must cease immediately," it said in a statement.
In Washington, General Vincent Brooks, commander of the US Army in the Pacific, said the United States had detected "increased militarization" of North Korea's nuclear program.
He said the increased range and precision of North Korean missiles were "of great concern" and represented a physical threat to US territory. He emphasized the need to conduct military exercises with South Korea, which have provoked increasingly heated North Korean rhetoric in recent days.
Brooks told the Center for Strategic and International Studies think tank it was "difficult to surmise exactly" where North Korea was in terms of its ability to weaponize its nuclear capability by mounting a warhead on a missile.
However he added: "It's difficult time, it's a dangerous time, and the potential for miscalculation is high."
North Korean Foreign Minister Ri Su Yong told the UN-backed Conference on Disarmament in Geneva on Tuesday that his country had the power to deter an "ever-increasing nuclear threat" by the United States with a pre-emptive strike.
Ri said the military exercises being staged by South Korea and the United States were "unprecedentedly provocative in nature and have an especially high possibility of sparking off a war."
North Korea fired two short-range missiles off its eastern coast on Monday, South Korean officials said, in a move seen as a response to the exercises, which Pyongyang regularly denounces as a preparation for war.
Brooks said in an interview with Reuters earlier on Wednesday that North Korea's demonstrations of increased capabilities emphasized the need for nations to cooperate on missile defense.
Brooks did not respond to a question at the think tank on whether he believed a prediction by a US research institute last month that Pyongyang could possess as many as 100 nuclear weapons within five years.
