TOKYO — Japan’s effort to contain the crisis at the Fukushima Dai-ichi nuclear plant suffered a setback, an official said early today, citing evidence that the reactor vessel of the No. 3 unit had been damaged.

The development, described at a news conference by Hidehiko Nishiyama, deputy director-general of the Japan Nuclear and Industrial Safety Agency, raises the possibility that radiation from the MOX fuel in the reactor — a combination of uranium and plutonium — could be released.

Nishiyama said the authorities believe that a breach might have occurred in the vessel, which houses the nuclear fuel. One sign of that took place on Thursday as three workers, who were trying to connect an electrical cable to an injection pump, were injured when they stepped into water that was found to be 10,000 times more radioactive than normal in a reactor.

The No. 3 unit, the only one of the six reactors at the site that uses the MOX fuel, was damaged by a hydrogen explosion on March 14. Workers have been seeking to keep it cool by spraying it with seawater, while the containment effort has focused on trying to restart the reactor cooling system.

The Japanese government will help people who wish to leave the area around the crippled Fukushima Dai-ichi Nuclear Power Plant, a government official said today.