The plant has leaked some low levels of radiation, but a breach could mean a much larger release of contaminants.
Suspicions of a possible breach were raised when two workers waded into water 10,000 times more radioactive 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.
Officials have previously said that small explosions at the reactor could have damaged it, but the high seepage of radiation could imply worse damage than previously believed.
A rupture in a reactor would mean a serious reversal following days of slow progress in containing radiation leaks.
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