Tokyo Electric Power Company says the No.1 reactor at the Fukushima Daiichi nuclear power plant is believed to be in a state of "meltdown".

The utility company said on Thursday that most of the fuel rods are likely to have melted and fallen to the bottom of the reactor. Earlier in the day, it found that the coolant water in the reactor is at a level which would completely expose nuclear fuel rods if they were in their normal position.

The company believes the melted fuel has cooled down, judging from the reactor's surface temperature.

But it suspects the meltdown created a hole or holes in the bottom of the reactor causing water to leak into the containment vessel.