The announcement that nuclear fuel partially melted at reactors No.2 and No.3 updated earlier information that reactor No.1 suffered a near complete core meltdown in the March disaster.

"The latest announcement means all three reactors with active fuel inside at the plant are now believed to have suffered fuel meltdowns in the wake of the devastating March 11 earthquake and ensuing tsunami," Kyodo said on its English-language page.

Government sources earlier hinted that meltdown occurred at the three reactors, but no official confirmation was available until Tuesday morning.

An earthquake and a tsunami that swept northeastern Japan two months ago damaged the cooling system at Fukushima, which resulted in serious radiation leaks. In mid-April, Japan's nuclear authorities assigned the highest level of danger to the Fukushima nuclear disaster for the first time after the devastating Chernobyl nuclear accident in the Soviet Union in 1986.

TEPCO expects the stabilization of the crippled reactors to take from six to nine months.