An explosion at the ageing Fukushima No. 1 atomic plant blew apart the building housing one of its reactors Saturday, a day after the biggest quake ever recorded in Japan unleashed a monster tsunami.

The atomic emergency escalated as crews struggled to prevent overheating at a second reactor where the cooling system has also failed, and the government warned that it too could suffer a blast.

Prime Minister Naoto Kan said the situation at the stricken power plant remained grave, and that Japan was facing its worst crisis since the end of World War II -- which left the defeated country in ruins.

"Whether we Japanese can overcome this crisis depends on each of us," said the premier, who was dressed in an emergency services suit.

AFP