The second nuclear blast in three days rocked Japan's earthquake-stricken Fukushima Dai-ichi power plant today, injuring 11 people and sending a massive column of smoke into the air.
The quake and subsequent tsunami have so far left at least 10,000 people dead, it's feared.
The country's chief cabinet secretary, Yukio Edano, said four army personnel and seven nuclear power plant workers were hurt when Unit 3 exploded.
Mr Edano said one of the workers was seriously injured but still conscious and the four military staff were only slightly hurt and had already returned to their unit.
Later in the day, another reactor at the plant lost its cooling capacity, raising the risk of another explosion.
The US said it had shifted its offshore forces away from the plant after detecting low-level radioactive contamination.
The aircraft carrier USS Ronald Reagan was about 100 miles offshore when it detected the radiation, which US officials said was about the same as one month's normal exposure to natural background radiation.
It was not clear if the radiation had leaked during today's explosion. It was felt 25 miles away, but the plant's operator said radiation levels at the reactor were still within legal limits.
The explosion at the plant's Unit 3, which authorities have been frantically trying to cool after a system failure in the wake of Friday's massive earthquake and tsunami, triggered an order for hundreds of people to stay indoors, said Mr Edano.
Operators knew an explosion was a possibility as they struggled to reduce pressure inside the reactor containment vessel, but apparently felt they had no choice if they wanted to avoid a complete meltdown. In the end, the hydrogen in the released steam mixed with oxygen in the atmosphere and set off the blast.
Tokyo Electric Power Co, which operates the plant, said radiation levels at Unit 3 were below levels where a nuclear operator must file a report to the government.
On Saturday, a similar explosion took place at the plant's Unit 1, injuring four workers and causing mass evacuations.
Shortly after today's explosion, Tokyo Electric warned it had lost the ability to cool Unit 2. Company officials Takako Kitajima said plant workers were preparing to inject seawater into the unit to cool the reactor, a move which could lead to an explosion there as well.
At least 1.4 million households had gone without water since the quake struck and some 1.9 million households were without electricity, partly due to the situation at the country's nuclear power stations.
People within a 12-mile radius were ordered to stay inside homes following today's blast, with eye-witnesses feeling the explosion 25 miles away.
More than 180,000 people have been evacuated from the area in recent days, and up to 160 may have been exposed to radiation after the first blast.
Also, Tokyo Electric Power held off on imposing rolling blackouts planned for today, but called for people try to limit electricity use. Mr Edano said the utility was still prepared to go ahead with power rationing if necessary.
The planned blackouts of about three hours each were meant to help make up for a severe shortfall after the key nuclear plants were left inoperable.