A hydrogen explosion has occurred at Japan's stricken Fukushima Dai-ichi nuclear plant, similar to one on Saturday at a different unit in the facility. Officials said the reactor was safe.

A massive column of smoke was seen belching from the plant's No 3 unit today.

The reactor had been under emergency watch for a possible explosion as pressure built up there following the first blast on Saturday in the facility's Unit 1.

Officials have been racing to stave off multiple reactor meltdowns after a devastating quake and tsunami incapacitated the Fukushima plant.

More than 180,000 people have been evacuated from the area, and up to 160 may have been exposed to radiation.

Chief cabinet secretary Yukio Edano said the reactor's inner containment vessel holding nuclear rods is intact, allaying some fears of the risk to the environment and public.

Mr Edano said people within a 12-mile radius were ordered inside their homes following today's blast.

Journalists felt the explosion 30 miles away.

The Tokyo Electric Power Corporation said three people were injured and seven were missing after today's explosion at a Japan nuclear plant.

The operator of the plant said radiation levels at the unit are within legal limits.

Tokyo Electric Power Co said radiation levels at Unit 3 of the stricken Fukushima Dai-Ichi plant were 10.65 microsieverts today, significantly under the 500 microsieverts at which a nuclear operator is legally bound to file a report to the government.