TOKYO (AP) - The operator of Japan's tsunami-hit nuclear plant says tons of highly radioactive water appears to have leaked into the ocean from a purification unit.

The leak comes as Tokyo Electric Power Co. struggles to keep the melted reactors cool and contain radiation and raises concerns about its ability to keep the plant stable. Similar leaks have occurred several times since last year, and officials say they do not pose an immediate health threat.

Workers spotted the leak early Thursday coming from a section of hose on a device used to decontaminate cooling water leaking from reactors. The company said it appeared to have stopped the leak.

The government declared in December the plant had achieved stability and radiation leaks have significantly decreased.