North Korea says it has for the first time successfully carried out uranium enrichment, a process that could give it a second way of making nuclear bombs.
Pyongyang already has a plutonium-based programme.
South Korea described the news as intolerable and accused the North of new threats and provocations, the BBC reports.
For many years North Korea denied American suspicions it was trying to enrich uranium.
But just three months ago, angered by the United Nations sanctions imposed as a result of its underground nuclear test, it announced it would begin the process.
North Korea's offical state media says experimental uranium enrichment has been successfully carried out.
The BBC reports the claim is impossible to verify, but if North Korea does have the ability, it would be arguably more worrying than the plutonium reprocessing used to create its existing atomic stockpile.