Whether the calculated revelation is a negotiating ploy by North Korea or a signal that it plans to accelerate its weapons program even as it goes through a perilous leadership change, it creates a new challenge for President Obama at a moment when his program for gradual, global nuclear disarmament appears imperiled at home and abroad. The administration hurriedly began to brief allies and lawmakers on Friday and Saturday — and braced for an international debate over the repercussions.

The scientist, Siegfried S. Hecker, a Stanford professor who previously directed the Los Alamos National Laboratory, said in an interview that he had been “stunned” by the sophistication of the new plant, where he saw “hundreds and hundreds” of centrifuges that had just been installed in a recently gutted building that had housed an aging fuel fabrication center, and that were operated from what he called “an ultra-modern control room.” The North Koreans claimed 2,000 centrifuges were already installed and running, he said.

American officials know that the plant did not exist in April 2009, when the last Americans and international inspectors were thrown out of the country. The speed with which it was built strongly suggests that the impoverished, isolated country, which tested its first nuclear device in 2006, had foreign help and evaded strict new United Nations Security Council sanctions imposed to punish its rejection of international controls.