TOKYO — Prime Minister Yoshihiko Noda of Japan has declared an end to the world’s worst nuclear crisis since Chernobyl, saying technicians have regained control of reactors at the Fukushima Daiichi nuclear power plant.

“Today, we have reached a great milestone,” Mr. Noda said in a televised address to the nation. “The reactors are stable, which should resolve one big cause of concern for us all.”

The declaration, nine months after a calamitous earthquake and tsunami set off a huge radiation leak, could set the stage for the return of more evacuees to affected areas.

But even before Mr. Noda’s announcement, some experts called the news premature, an attempt to quell continuing public anger over the accident and paper over remaining threats to the plant. The experts argue that the devastated plant remains vulnerable to large aftershocks, which could knock out the jury-rigged cooling system that helped workers bring the reactors into a relatively stable state known as a “cold shutdown.”