From bad to worse.

TOKYO — A suspected breach in the reactor core at one unit of a stricken Fukushima nuclear plant could mean more serious radioactive contamination,Japanese officials said Friday, revealing what may prove a major setback in the mission to bring the leaking plant under control.

The uncertain situation halted work at the Fukushima Dai-ichi nuclear complex, where dozens had been working feverishly to stop the overheated plant from leaking dangerous radiation, officials said. The plant has leaked some low levels of radiation, but a breach could mean a much larger release of contaminants.

Previous radioactive emissions have come from intentional efforts to vent small amounts of steam through valves to prevent the core from bursting. However, releases from a breach could allow uncontrolled quantities of radioactive contaminants to escape into the surrounding ground or air.