Japan was unprepared for a severe nuclear accident like the tsunami-caused Fukushima disaster and its damage was greater than previously thought, the country acknowledged Tuesday in a report being submitted to the UN nuclear agency.

In the report, the government said the core melted in three units and likely breached the inner containment vessels after the March 11 tsunami knocked out the Fukushima Dai-ichi nuclear plant's power and cooling systems. Fuel at Unit 1 started melting hours earlier than previously estimated.

The report, compiled by Japan's nuclear emergency taskforce, factors in a preliminary evaluation by a team from the International Atomic Energy Agency and will be submitted to the IAEA.

The report acknowledged a lack of independence at Japan's nuclear regulator and promised to improve the safety control system.

The report comes the day after the government's nuclear officials doubled the estimate of how much radiation leaked from a tsunami-hit nuclear plant and said the damage to the reactors was greater than previously thought.

The Nuclear and Industrial Safety Agency said Monday in a report that nuclear fuel inside three reactors at the Fukushima Dai-ichi plant probably melted through not only the reactor cores but also through the inner containment vessels.

The report Monday said twice as much radiation may have been released into the air as earlier estimated by NISA. That would be about one-sixth of the amount released at Chernobyl instead of the earlier estimate of one-tenth.

NISA said its analysis used a different method than had been employed by the plant's operator last month and is believed to "better reflect reality."