Workers of the operator, Tokyo Electric Power Company (TEPCO), are engaged in moving highly radioactive water from the tunnel of the No. 2 reactor to a temporary storage facility.
However, the power company said the water level in the tunnel of the No. 3 reactor rose to 99 centimetres below the surface last evening. That passes the level at which TEPCO plans to remove the water, but it is yet to secure storage space, national broadcaster NHK reported.
The water level in the basement of the No.3 reactor's turbine building also rose by 10 centimetres over 3 days, more than a month after a magnitude-9 quake and tsunami left nearly 30,000 people dead or unaccounted for in Japan's north east.
Earlier, a survey found an increase in the density of radioactive substances in the water in the basement of the No.
4 reactor's turbine building. The water level in the No. 4 reactor's turbine building rose by 20 centimetres in 10 days.
TEPCO said the levels of cesium-134 and 137 increased about 250-fold and iodine-131 increased about 12 times compared with the levels one month ago.
Contamination of this level requires the TEPCO workers to prioritise the transfer or disposal of the water.
TEPCO said the water being used to cool the No. 3 reactor could be leaking into No. 4 as their turbine buildings are connected.
The government's nuclear agency separately said that water may be leaking from the No. 1 reactor container of the crisis-hit Fukushima plant, and that remote-controlled robots are expected to check the situation inside the reactor building, Kyodo news agency reported.
The power company has also rewired the power grid at the Fukushima Daiichi nuclear power plant to secure supply of electricity in case of another strong quake.
The company completed work to connect the cables for the No.1 and No.2 reactors to the grid for the No.5 and No.6 reactors last evening. The plant's six reactors had been supplied with electricity in pairs from external power sources.
The work is aimed at ensuring that if any one of the three outside sources is cut off, the others can be used to cool the reactors.
During the operation, external power to the No.1, No.2 and No.5 reactors was suspended for a few hours, but there were no problems.