Washington has warned Pyongyang to refrain from ''irresponsible provocation'' after North Korea announced that the country's main nuclear facility was up and running on Tuesday after eight years of deactivation.
The White House said North Korea should ''focus instead on fulfilling its international obligations''.
The reactor at Yongbyon is the source of plutonium for North Korea's nuclear weapons programme. It was shut down in 2007 as part of a deal to receive international aid but is now reportedly running normally. Experts believe that the facility is capable of creating one nuclear bomb's worth of plutonium every year.
Reports out of Pyongyang claim that North Korea has created a nuclear warhead capable of being launched at its enemies.
The international community will not accept North Korea as a nuclear state, White House spokesman Josh Earnest said.
With a big anniversary drawing near, North Korea declared on Tuesday it has upgraded and restarted all of its atomic fuel plants - meaning it could possibly make more, and more sophisticated, nuclear weapons.
The statement, coming just a day after it said it is ready to conduct more rocket launches any time it sees fit, has heightened concerns the North may soon either conduct a launch - which Washington and its allies see as a pretext for testing missile technology - or hold another test of nuclear weapons that it could conceivably place on such a rocket.
Either would be sure to get world attention and be milked by North Korea's state media as major achievements by Kim Jong Un and his ruling regime.
North Korea has spent decades trying to develop operational nuclear weapons. It is thought to have a small arsenal of atomic bombs and an impressive array of short- and medium-range missiles. But it has yet to demonstrate that it can produce nuclear bombs small enough to place on a missile, or missiles that can reliably deliver their bombs to faraway targets.