An explosion was heard and white smoke was reportedly seen at Japan's quake-hit Fukushima No.1 nuclear plant today, Jiji press agency said.

Several workers were reported to be injured in the explosion, and smoke was seen billowing out of the plant.

The Japan Nuclear Safety Agency later said however the explosion was not at the reactor.

Radioactivity at the plant was 20 times over the normal level, and Japan's Nuclear Safety Commission has said it may be experiencing meltdown.