The operator of Japan's damaged Fukushima nuclear plant has acknowledged it failed for two months to announce that meltdowns had occurred in the cores of three of the reactors.

Tokyo Electric Power Co. said Thursday its officials were unaware of a company emergency manual that defined a meltdown as damage exceeding 5 percent of a reactor's fuel.

Instead, TEPCO described the condition of the reactors as less serious "core damage" for two months after the plant was hit by a massive earthquake and tsunami in March 2011, despite damage estimated to range from 25 to 55 percent.

TEPCO said its initial wording may have been misleading, but didn't affect its emergency response.

It said it discovered the emergency manual this month, and promised an investigation into why it was overlooked.