The nuclear group's new evaluation, released Monday, goes further than previous statements in describing the extent of the damage caused by an earthquake and tsunami on March 11.
The announcement will not change plans for how to stabilize the Fukushima Daiichi plant, the agency said.
Reactors 1, 2 and 3 experienced a full meltdown, it said.
The plant's owner, Tokyo Electric Power Co., admitted last month that nuclear fuel rods in reactors 2 and 3 probably melted during the first week of the nuclear crisis.
It had already said fuel rods at the heart of reactor No. 1 melted almost completely in the first 16 hours after the disaster struck. The remnants of that core are now sitting in the bottom of the reactor pressure vessel at the heart of the unit and that vessel is now believed to be leaking.....read on