No. 3 unit of the Fukushima Daiichi Nuclear Power Plant, is seen, 2nd from right, with unit 1 reactor, left, with its top part of walls blown off after Saturday's explosion seen in Okumamachi, Fukushima Prefecture (state) , northern Japan Monday, March 14, 2011. Japanese officials say they believe a hydrogen explosion has occurred at the nuclear plant, similar to an earlier one at a different unit in the facility. (AP Photo/Kyodo News) JAPAN OUT, MANDATORY CREDIT, NO SALES IN CHINA, HONG KONG, JAPAN, SOUTH KOREA AND FRANCE

TOKYO - Japanese officials say they believe a hydrogen explosion has occurred at Japan's stricken Fukushima Dai-ichi nuclear plant, similar to an earlier one at a different unit in the facility.

A massive column of smoke was seen belching from the plant's No. 3 unit Monday. The reactor had been under emergency watch for a possible explosion as pressure built up there following a hydrogen blast Saturday in the facility's Unit 1.

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

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

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