The US has called for North Korea to abandon its nuclear ambitions and said it would be "extremely alarming" if Pyongyang follows through on a vow to restart its plutonium reactor.
The White House and the State Department said they were taking seriously an almost daily string of threats from North Korea toward the US and South Korea, ratcheted up a notch when the North said it would revive its long-dormant reactor and ramp up production of nuclear weapons material.
But officials cast doubt on whether North Korea would follow through, portraying the latest threat as part of a pattern of antagonistic taunts that, so far, have not been backed up by action.
"There's a long way to go between a stated intention and actually being able to pull it off," said State Department spokeswoman Victoria Nuland.
Still, the US is taking steps to ensure it has the capacity to defend itself and its allies against any threats from North Korea, and President Barack Obama is being updated regularly, said the president's spokesman, Jay Carney.
"The entire national security team is focused on it," Mr Carney said.
North Korea said that scientists will quickly begin "readjusting and restarting" the facilities at its main Nyongbyon nuclear complex, which was shuttered as part of international nuclear disarmament talks in 2007 that have since stalled. Officials said those operations would include the plutonium reactor and a uranium enrichment plant, both of which could produce fuel for nuclear weapons.
Mr Carney called the North's announcement a violation of its international obligations and said that while North Korea has obtained nuclear weapons in the past, it has not tempered the US resolve to see the Korean peninsula rid of nuclear weapons.
He called on Russia and China, two countries he said have influence on North Korea, to use that influence to persuade the North to change course.
North Korea's recent tide of nuclear vows and aggressive threats are seen as efforts to force Washington into disarmament-for-aid talks and to boost young leader Kim Jong Un's stature as a strong military leader. Pyongyang has reacted angrily to US-South Korean military drills and a new round of UN and US sanctions that followed North Korea's February 12 underground nuclear test.