Japan's Self-Defense Forces (SDF) and firefighting personnel resumed shooting water at the Fukushima Daiichi nuclear power plant early Monday morning in an effort to cool down reactors and overheating spent fuel pools.

External cooling efforts began at the plant's No. 4 reactor on Sunday and since Thursday the SDF and firefighting personnel have dumped and sprayed some 3,700 tons of water on the stricken No. 3 reactor.

Tokyo Electric Power Co. (TEPCO), owner and operator of the plant, said that efforts to connect the power-receiving facilities at the No. 2 and No. 5 reactors had been successful on Sunday, but noted that multiple component systems must be restored before the reactors become fully operational.

The utility said that at the time of the quake, the No. 1, No. 2 and No. 3 reactors at the Daiichi facility were the only ones operating and shutdown automatically as they are supposed to.

But due to cooling function failures in the reactors, some of the cores are believed to have partially melted, TEPCO said.

The buildings housing the No. 1, No. 3 and No. 4 reactors have been severely damaged, TEPCO said, and fuel pools in the reactors have been left uncovered.

But in the No. 4 reactor some of the fuel was not in the reactor core at the time of the quake but in the spent-fuel pool, which also lost its cooling function and lost the roof of its building.

Nuclear experts said the No. 3 reactor is potentially the most dangerous as its fuel is made up of plutonium-uranium mixed oxide (MOX), and poses the greatest risk of releasing highly toxic plutonium in the event of a meltdown.

Added to this, the fuel in the reactors is uranium, they said.

The No. 2 reactor's containment vessel, meanwhile, suffered damage to its pressure-suppression chamber, TEPCO said.

A magnitude 9.0 quake and ensuing tsunami knocked out power on March 11 at the nuclear facility on the Pacific coast of Fukushima Prefecture, 220 km northeast of Tokyo.

The No. 1, No. 2 and No. 3 reactors, which were operating at the time of the quake halted automatically and subsequently lost their cooling functions.