Japan's Tokyo Electric Power Co. (TEPCO) on Tuesday connected the No. 4 or the fifth reactor at the quake-hit Fukushima No. 1 nuclear power plant to an external power supply, as smoke is seen rising from the No. 2 and No. 3 reactor buildings.

As was the case with Monday's sighting of smoke billowing from the same building, the smoke seen at the No. 2 reactor is likely to be steam and the smoke at the No. 3 reactor caused by burning rubble, Japan's defense minister said.

But while no explosions in the vicinity were heard, the International Atomic Energy Agency officials said Monday that levels of radiation up to 1,600 times higher than normal were detected in an area 20 kilometers away from the troubled nuclear plant.

According to the IAEA, radiation levels of 161 microsievert per hour were detected in the town of Namie in Fukushima Prefecture. The government maintained its official exclusion zone of a 20 kilometer radius from the plant and advised people within a 20-30 kilometer radius to stay indoors and switch off air conditioning.

The utility has been battling to get the six reactors at the plant back online following the March 11 magnitude 9.0 megaquake and tsunami that knocked out the plant's critical cooling systems and led to partial meltdowns of some of the reactors' cores.

TEPCO said however that it had managed to connect its No. 1 reactor on Monday night to an external power source.

The utility said the No. 1, No. 2, No. 5 and No. 6 reactors are now all set to receive power from independent sources which is a critical first step to revive reactors' cooling systems.

But due to the radioactive substances being passed through the plant's discharging outlets, TEPCO announced Monday that highly concentrated radioactive substances have been detected in seawater close to the plant.

According to TEPCO, radioactive iodine-131 was detected in seawater samples at levels 126.7 times higher than the legal concentration limit.

Levels of cesium-134 registered 24.8 times higher and those of cesium-137 16.5 times higher.

Meanwhile, trace amounts of cobalt 58 were also detected in seawater samples.

Prior to the March 11 quake and ensuing tsunami, no radioactive substances were detected in the seawater near the plant, the utility said.

Industry minister Banri Kaieda said Monday that the situation remains "extremely tough."

"It is difficult to say that things are showing progress from my feeling," said the head of the economy, trade and industry ministry, to which the Nuclear and Industrial Safety and Agency belongs.