A huge explosion hit another reactor at an earthquake-damaged Japanese nuclear power plant early on Tuesday, the third blast since Saturday, the plant operator said.
"There was a huge explosion" between 6:00 am (2100 GMT Monday) and 6:15 am at the number-two reactor of Fukushima No 1 nuclear power plant, a Tokyo Electric Power Co (TEPCO) spokesman said.
The government also reported apparent damage to part of the container shielding the same reactor at Fukushima 250 kilometres northeast of Tokyo, although it was unclear whether this resulted from the blast.
Chief cabinet secretary Yukio Edano told reporters the suppression pool of the number-two nuclear reactor appeared to have been damaged.
This is the bottom part of the container, which holds water used to cool it down and control air pressure inside.
"But we have not recorded any sudden jump in radiation indicators," Edano added.
Japan is frantically working to avert a nuclear meltdown after Friday's massive earthquake and tsunami cut power to the ageing plant and knocked out cooling systems.
On Saturday an explosion blew apart the building surrounding the plant's number-one reactor but the seal around the reactor itself remained intact, officials said.
On Monday, shortly after Prime Minister Naoto Kan said the plant was still in an "alarming" state, a blast at its number-three reactor shook the facility, injuring 11 people and sending plumes of smoke billowing into the sky.
Late on Monday TEPCO said fuel rods at the number-two reactor were almost fully exposed after a cooling pump there temporarily failed.
The UN's nuclear watchdog, the International Atomic Energy Agency, said Tokyo had asked for expert assistance.
But agency chief Yukiya Amano moved to calm global fears that the situation could escalate to match the world's worst nuclear crisis at Chernobyl in the Ukraine in 1986.
Japan asks for more frequent local radiation checks: Kyodo
Japan's science minister asked local governments to make more frequent radiation checks, Kyodo news agency said. The agency also said there had been one explosion at the No. 4 reactor at a stricken plant in the northeast. (Reuters)