Technicians are battling to stabilise a third reactor at a quake-stricken Japanese nuclear plant that has been rocked by a second blast in three days.
Sea water is being pumped into reactor 2 at the Fukushima Daiichi plant after its fuel rods were fully exposed twice.
International nuclear watchdogs said there was no sign of a meltdown but one minister said a melting of rods was "highly likely" to be happening.
The crisis was sparked by Friday's 9.0-magnitude quake and tsunami.
Thousands of people are believed to have died in the earthquake and tsunami, and millions are spending a fourth night without water, food, electricity or gas. More than 500,000 people have been left homeless.
'All our effort'
On Monday a hydrogen blast at the Fukushima Daiichi's reactor 3 injured 11 people and destroyed the building surrounding it. The explosion was felt 40km (25 miles) away and sent a huge column of smoke into the air.
It followed a blast at reactor 1 on Saturday.
Chief Cabinet Secretary Yukio Edano said there were signs that the fuel rods were melting in all three reactors.
"Although we cannot directly check it, it's highly likely happening," he told reporters.
Both explosions at the plant were preceded by cooling system breakdowns but the Tokyo Electric Power Company (Tepco) said neither blast penetrated the thick containment walls shielding the reactor cores.
It said radiation levels outside were still within legal limits.
But shortly after Monday's blast, Tepco warned it had lost the ability to cool Fukushima Daiichi's reactor 2.
Officials battled all Monday and into the early hours of Tuesday to try to keep water levels up in order to cool the nuclear fuel rods, but on two occasions the rods have been fully exposed.
Exposure for too long a period of time can damage the rods and raise the risk of a meltdown.
Four of the five pumps used to administer cooling sea water were believed to have been damaged by the blast at reactor 3.
Nuclear and Industrial Safety Agency official Ryohei Shiomi said reactors 1 and 3 had "somewhat stabilised" but "unit 2 now requires all our effort".
A Tepco official later pointed to some improvement and said the company did "not feel that a critical event is imminent".
Pressure has been released from the containment vessel, reducing the risk of a catastrophic explosion, but if the vessel is cracked it could still release radioactive material.
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 offshore.
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 miles) to the south.
The Japanese government has asked the International Atomic Energy Agency (IAEA) to send a team of experts to help.
"Nuclear plants have been shaken, flooded and cut off from power. Operators have suffered personal tragedies," IAEA director general Yukiya Amano told reporters in Vienna. "[But] the reactor vessels have held and radioactive release is limited."
Mr Amano said the crisis was very unlikely to turn into another Chernobyl, the nuclear power plant in Ukraine that blew up in 1986.
James Lyons, a senior IAEA nuclear safety official, also said: "I think at this time we don't have any indication of fuel... currently melting."
But the French Nuclear Safety Authority (ASN) cast doubt on Japan's classification of the crisis at Fukushima as level 4 of 7 on the International Nuclear Event Scale. Chernobyl was classified as level 7.
"Level four is a serious level," ASN chief Andre-Claude Lacoste said, but added: "We feel that we are at least at level five or even at level six."
Meanwhile, the relief operation is continuing in the north-east.
About 2,000 bodies were found washed ashore on beaches in Miyagi prefecture, police said.
About 1,000 were found on the Ojika peninsula and another 1,000 in the town of Minamisanriku, which was flattened by the tsunami.
The BBC's Rachel Harvey says the valley where Minamisanriku once stood is now just a scene of complete devastation.
Everything was flattened by the force of the wave, with only the town's hospital and a government building remaining, our correspondent says. Apart from that, there are now just piles of debris. A couple stood in the midst of the destruction, staring in disbelief, she adds.
The official death toll stands at nearly 1,900 but officials in Miyagi have estimated that 10,000 people died in the prefecture alone.
Thousands are still unaccounted for - including hundreds of tourists - while many remote towns and villages remain cut off and have had no help since Friday's earthquake.
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.
"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 area 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.