SEOUL, South Korea — North Korean scientists are able to build crucial equipment for uranium-based nuclear bombs on their own, cutting the need for imports that had been one of the few ways outsiders could monitor the country’s secretive atomic work, according to evidence gathered by two American experts.

The experts say materials published in North Korean scientific publications and news media show that Pyongyang is mastering domestic production of essential components for the gas centrifuges needed to make such bombs. The development further complicates long-stalled efforts to stop a nuclear bomb program that Pyongyang has vowed to expand, despite international condemnation.

If Pyongyang can make crucial centrifuge parts at home, outsiders can’t track sensitive imports. That could spell the end of policies based on export controls, sanctions and interdiction that have been the centerpiece of international efforts to stop North Korea’s nuclear program over the last decade, Joshua Pollack, a Washington-based expert on nuclear proliferation, said in remarks prepared for delivery Wednesday at a Seoul symposium and provided in advance

“If they’re not importing these goods in the first place, then we can’t catch them in the act,” said Pollack, who gathered the evidence with Scott Kemp, an expert on centrifuge technology at the Massachusetts Institute of Technology.

The state of North Korea’s nuclear program is of vital concern to Washington because Pyongyang wants to build an arsenal of nuclear-armed missiles that can reach American shores.