A second explosion has hit a Japanese nuclear plant that was damaged in Friday’s earthquake, but officials said the reactor core was still intact.
A huge column of smoke billowed from Fukushima Daiichi’s reactor 3, two days after a blast hit reactor 1.
The latest explosion, said to have been caused by a hydrogen build-up, injured 11 people, one of them seriously.
Soon afterwards, the government said a third reactor at the plant had lost its cooling system.
Water levels were now falling at reactor 2, which is to be doused with sea water, said government spokesman Yukio Edano.
A similar cooling system breakdown preceded the explosions at reactors 1 and 3.
Technicians have been battling to cool reactors at the Fukushima Daiichi plant since Friday, following the quake and tsunami.
Japanese police have so far confirmed 1,597 deaths, but the final toll is expected to be much higher. Tens of thousands of people have been evacuated from the area around Fukushima Daiichi plant.
At least 22 people were said to be undergoing treatment for radiation exposure.
The government said radiation levels were below legal limits after Monday’s explosion. Tokyo Electric Power, which operates the plant, said the reactor’s containment vessel had resisted the impact.
Experts say a disaster on the scale of Chernobyl in the 1980s is highly unlikely because the reactors are built to a much higher standard and have much more rigorous safety measures.
Earlier, the prime minister said the situation at the nuclear plant was alarming, and the earthquake had thrown Japan into “the most severe crisis since World War II”.
The government advised people not to go to work or school on Monday because the transport network would not be able to cope with demand.
The capital Tokyo is also still experiencing regular aftershocks, amid warnings that another powerful earthquake is likely to strike very soon.
Meanwhile, tens of thousands of relief workers, soldiers and police have been deployed to the disaster zone. Preliminary estimates put repair costs from the earthquake and tsunami in the tens of billions of dollars. The disaster is a huge blow for the Japanese economy (the world’s third largest), which has been ailing for two decades.