"All the functions to keep cooling water levels in No. 3 reactor have failed at the Fukushima No. 1 plant,'' operator TEPCO said, adding that pressure was rising slightly.
Kyodo reported that the fuel rods at one reactor were now three metres above the water, and that a radiation leak believed to be from the reactor itself had now reached levels above the legal limit.
Japan's ambassador to the US Ichiro Fujisaki told CNN: ``There was a partial melt of a fuel rod, melting of fuel rod. There was a part of that ... but it was nothing like a whole reactor melting down.''
Japan's nuclear safety agency rated the incident at four on the international scale of zero to seven. The 1979 Three Mile Island accident in the United States was rated five, while the 1986 Chernobyl disaster was a seven.
The International Atomic Energy Agency said about 200,000 people had so far been evacuated from the area around the two Fukushima plants. There are a total of 10 reactors at the two plants.
Media reports said three residents - bedridden patients evacuated from a hospital near the No. 1 plant - had been found to be exposed to radiation after spending a long time outdoors awaiting rescue.
US nuclear experts warned that pumping sea water to cool the reactor was an ``act of desperation'' that, in the worst-case scenario, may foreshadow a Chernobyl-like disaster.
Several experts, in a conference call with reporters, also predicted that regardless of the outcome of the atomic plant crisis, the accident will seriously damage the nuclear power renaissance.