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 Friday at the Fukushima Dai-ichi nuclear complex, where dozens had been working feverishly to stop the overheated plant from leaking dangerous radiation, officials said.

Suspicions of a possible breach were raised when two workers waded into water 10,000 more radiactive than normal and suffered skin burns, the Nuclear and Industry Safety Agency said.

However, though damage cannot be ruled out, the cause remained unclear, spokesman Hidehiko Nishiyama told reporters.

"It is possible there may be damage somewhere in the reactor," he said, adding later that there was no data suggesting there were any cracks and that a leak in the plumbing or the vents could be to blame.

The confusion was yet another setback to the urgent task of gaining control of the Fukushima nuclear plant 140 miles (220 kilometers) northeast of Tokyo two weeks after a magnitude-9 quake triggered a tsunami that engulfed the facility and knocked out its crucial cooling system.

The plant has been releasing radiation, with elevated levels of radiation turning up in raw milk, seawater and 11 kinds of vegetables, including broccoli, cauliflower and turnips.

Tap water in several areas of Japan — including Tokyo — also tested with radiation levels considered unsafe for infants, who are particularly vulnerable to cancer-causing radioactive iodine, officials said.

The scare caused a run on bottled water in the capital, and prompted city officials to distribute bottled water to families with babies.