The second hydrogen explosion in three days has rocked Japan's stricken Fukushima Dai-ichi nuclear plant, sending a massive column of smoke into the air and wounding six workers.
The explosion at the plant's Unit 3, which authorities have been frantically trying to cool following a system failure in the wake of a massive earthquake and tsunami, triggered an order for hundreds of people to stay indoors, said Chief Cabinet Secretary Yukio Edano.
The blast follows a similar explosion on Saturday that took place at the plant's Unit 1, which injured four workers and caused mass evacuations.
Japan's nuclear safety agency said six workers were injured in Monday's explosion but it was not immediately clear how, or whether they were exposed to radiation. They were all conscious, said the agency's Ryohei Shomi.
The operator of the plant said radiation levels at the unit are within legal limits.
Earlier, Tokyo Electric Power Co said three workers were injured and seven missing.
The reactor's inner containment vessel holding nuclear rods was intact, Edano said, allaying some fears of the risk to the environment and public. TV footage of the building housing the reactor appeared to show similar damage to Monday's blast, with outer walls shorn off, leaving only a skeletal frame.
More than 180,000 people have evacuated the area in recent days, and up to 160 may have been exposed to radiation.
Earlier on Monday, pressure had jumped inside Unit 3, forcing the evacuation of 21 workers. But they returned to work after levels appeared to ease.
© Press Association