An explosion was heard at the quake-hit nuclear plant of Japan on Saturday, soon after speculations of melt-down were made following the failure of the cooling system of the plant due to Friday's powerful earthquake and tsunami.

As white smoke billowed out of Japan's Fukushima 1 nuclear power plant, Japanese officials said they feared the reactor could melt down.

Several workers were reported injured in the explosion, and the external radiation is reportedly increased 20 times than normal, local media said.

While local doctors advise people to stay indoors, officials said the actual reason of the explosion was still unknown and investigation is on.

We have faced more than 150 quakes since yesterday. Tsunami warning has been toned down, but the major worry now is aftershock and nuclear radiation leaks, Sheetal Prakash, an Indian residing in Tokyo, wrote on Facebook, indicating that the earthquake-torn country is presently threatened by the risks of a nuclear meltdown and radiation risks.