Technicians are battling to stabilise a third reactor at a quake-stricken Japanese nuclear plant, which has been rocked by a second blast in three days.
The fuel rods inside reactor 2 at the Fukushima Daiichi plant have been fully exposed on two separate occasions, raising fears of a meltdown.
Sea water is being pumped into the reactor to try to prevent overheating.
A cooling system breakdown preceded explosions at the plant's reactor 3 on Monday and reactor 1 on Saturday.
The latest hydrogen blast injured 11 people, one of them seriously. It was felt 40km (25 miles) away and sent a huge column of smoke into the air.
The outer building around the reactor was largely destroyed.
But as with the first explosion, Tokyo Electric Power Company (Tepco) said the thick containment walls shielding the reactor cores remained intact. It also said radiation levels outside were still within legal limits.
'Not optimistic'
Shortly after the blast, Tepco warned that it had lost the ability to cool Fukushima Daiichi's reactor 2.
Hours later, the company revealed that the fuel rods inside had been exposed fully at one point, reportedly for about two-and-a-half hours. It said a fire pump that had been used to pump seawater into the reactor had run out of fuel.
The company is now trying to inject sea water into the reactor to cover the fuel rods, cool them down and prevent another explosion.
Initially, water levels continued to fall despite the efforts, as only one of the five fire pumps was working, officials said. The other four were believed to have been damaged by the blast at reactor 3.
By Monday evening, the water level inside the reactor had risen to 2m. But later, Tepco officials said the fuel rods had again been fully exposed.
Air pressure inside reactor 2 rose suddenly when the air flow gauge was accidentally turned off. That blocked the flow of water into the reactor, leading to the water level dropping and the rods being exposed at about 2300 local time (1400 GMT).
"We are not optimistic but I think we can inject water once we can reopen the valve and lower the air pressure," a Tepco official told reporters, according to the AFP news agency.
Exposure for too long a period of time can damage the fuel rods and raise the risk of overheating and possible meltdown.
Japan's Nuclear and Industrial Safety Agency said workers were also battling rising pressure within the reactor. They have opened vents in the containment vessel, which could release small amounts of radiation.
Chief Cabinet Secretary Yukio Edano said there were signs that the fuel rods were melting in all three reactors at Fukushima Daiichi.
"Although we cannot directly check it, it's highly likely happening," he told reporters.
Chief Cabinet Secretary Yukio Edano said the core container at the reactor was still intact
However, he insisted that radiation around the plant remained at tolerable levels.
Nevertheless, nearly 185,000 people have been evacuated from a 20km (12 mile) exclusion zone around the plant.
The US said it had moved one of its aircraft carriers from the area after detecting low-level radiation 160km (100 miles) offshore.
Experts say a disaster on the scale of Chernobyl in the 1980s is highly unlikely because the reactors are built to a higher standard and have much more rigorous safety measures.
Earlier, Tepco said it had restored the cooling systems at two of the three reactors experiencing problems at the nearby Fukushima Daini power plant, 11.5km (7.1 miles) to the south.
The Japanese government has asked the International Atomic Energy Agency (IAEA) to send a team of experts to help, the UN agency has said.
Complete devastation
Meanwhile, the relief operation is continuing after Friday's magnitude 9.0 earthquake, which triggered a tsunami that devastated swathes of the north-eastern coast of the country.
About 2,000 bodies had been found washed ashore on beaches in the north-eastern prefecture of Miyagi, police said.
A thousand were found on the Ojika peninsula and another thousand in the town of Minamisanriku, which was flattened by the tsunami.
The BBC's Alastair Leithead is near the town of Minamisanriku which has been virtually flattened
The BBC's Rachel Harvey says the valley where Minamisanriku once stood is now just a scene of complete devastation.
Everything in the tsunami's path was flattened, with only the town's hospital and a government building remaining, our correspondent says. Apart from that, she adds, there are just a few rooftops here and there.
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People are surviving on little food and water. Things are simply not coming”
End Quote Hajime Sato Iwate prefectural government
The total number of people killed by the disaster remains unclear. The official death toll stands at nearly 1,900, but officials in Miyagi have estimated that 10,000 people died in the prefecture alone.
Tens of thousands are still unaccounted for, while many remote towns and villages remain cut off and have had no help since Friday's earthquake.
Japanese news agency Kyodo reported that more than 500,000 people had been displaced by the earthquake, tsunami and nuclear emergency.
Millions of others have spent three nights without water, food or heat in near-freezing winter weather.
The government has deployed 100,000 troops to lead the aid effort. They have been given 120,000 blankets, 120,000 bottles of water, tonnes of food, and 111,000 litres (29,000 gallons) of petrol to distribute.
But Hajime Sato, a government official in Iwate prefecture, one of the three hardest hit, said it had received so far only 10% of the food and other supplies they had requested from the central government.
Elderly people sleep on the floor at an evacuation centre set in a gymnasium in Kawamata, Fukushima prefecture (14 March 2011) More than 500,000 people are believed to have been displaced from their homes
"People are surviving on little food and water. Things are simply not coming," he told the Associated Press.
Communications networks are also down in many areas.
The government asked 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 still experiencing regular aftershocks, amid warnings that another powerful earthquake is likely to strike very soon.
A 6.2-magnitude tremor on Monday triggered a new tsunami scare on the Pacific coast, with the authorities telling people to flee to higher ground.
The Foreign Office has updated its travel advice to warn against all non-essential travel to Tokyo and north-eastern Japan. British nationals and friends and relatives of those in Japan can contact the Foreign Office on +44(0) 20 7008 0000.
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