*IAEA official Graham Andrew told reporters in Vienna that the situation at Fukushima had not deteriorated, but could yet do so.
Japanese military helicopters have dropped four loads of water on the plant, trying to avert a full meltdown. Plant operators say they are trying to finish a power line which could bring the plant's cooling systems back online.
Some US military personnel in Japan have received potassium iodide tablets to help counter any radiation exposure and the US has urged American citizens living within 80km (50 miles) of the Fukushima complex to leave; Japan's own exclusion zone is only 20km.
The number of confirmed dead and missing now stands at nearly 13,000 while some 450,000 people have been staying in temporary shelters amid sub-zero night-time temperatures.
In Spite Of the crisis in Japan, the yen has risen close to its highest value against the US dollar since World War II. The BBC business reporter says the yen is being driven by market belief that Japan will need to repatriate some of its huge foreign holdings to pay for reconstruction.
Live page reporters: Alexandra Fouché, Patrick Jackson, Olivia Lang, Matthew Davis and Stephanie Holmes
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*********Japan says it is stepping up efforts to cool overheating fuel at the tsunami-hit Fukushima Daiichi nuclear plant
Helicopters and water cannon have dumped seawater to try to prevent fuel rods melting, as engineers try to restore power to the coolant pumps.
The International Atomic Energy Agency says the situation remains serious but has not significantly worsened.
The confirmed death toll from Friday's 9.0 magnitude quake, which triggered the tsunami, has risen above 5,400.
Police say about 9,500 people are still missing.
IAEA official Graham Andrew told reporters in Vienna that the situation at Fukushima had not deteriorated, but could yet do so.
"We could say it's reasonably stable at the moment compared to yesterday," he said.
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