* Japan facing "extremely difficult circumstances" - Amano
* Chernobyl-style disaster "very unlikely"
* Japan has requested IAEA expert missions
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By Fredrik Dahl and Michael Shields
VIENNA, March 14 Japan's massive earthquake and tsunami shook and flooded nuclear power plants but left reactor vessels intact and radiation release was limited, the U.N. atomic watchdog chief said on Monday.
Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), expressed confidence Japanese authorities were doing all they could to restore safety at the sites and said a Chernobyl-style disaster was "very unlikely."
He spoke as Japan scrambled to avert a meltdown at a stricken nuclear complex after a hydrogen explosion at one reactor and exposure of fuel rods at another, just days after the devastation that killed thousands.
"The Japanese authorities are working as hard as they can, under extremely difficult circumstances, to stabilise the nuclear power plants and ensure safety," Amano told the agency's first news conference since Friday's earthquake.
"The nuclear plants have been shaken, flooded and cut off from electricity," he said. But "the reactor vessels have held and radioactive release is limited."
His deputy for nuclear safety, Denis Flory, said radiation levels at the Fukushima plant peaked on Saturday but then fell. "We are in a range which is not an indication of high risk," Flory said.
Amano, a veteran Japanese diplomat, said Japan had asked the U.N. agency to provide expert missions.
Fukushima, 240 km (150 miles) north of Tokyo, has suffered explosions at two of its reactors, on Saturday and on Monday, which sent huge plumes of smoke billowing above the plant.
The nuclear accident in which cooling systems broke down after the quake and tsunami, was the worst since the Chernobyl disaster in Ukraine in 1986, provoking fears of a fuel meltdown and a major radiation leak.
In Chernobyl, the explosion took place because the nuclear reaction was not halted, unlike in Fukushima, where the reactor was automatically shut down when the earthquake occurred.
NO INDICATION OF MELTDOWN
The Japanese authorities faced criticism at home for being ill-prepared.
Amano said Japanese experts were doing their best to stabilise the reactors and "we will not try to second-guess the people on the ground." He did not say the situation was under control.
Citing design and structural differences in the plants, he dismissed any comparisons to Chernobyl.
"This is not an accident because of human errors or design, this is because of a huge natural catastrophe ... this reactor is designed so that it can (withstand) severe accidents."
The Fukushima plant operator, Tokyo Electric Power Co (TEPCO) (9501.T), said fuel rods at the plant's No. 2 reactor were fully exposed, which could lead to the rods melting down.
There were earlier reports of partial fuel rod meltdowns at both the No. 1 and the No. 3 reactors, where the explosions had occurred, but a senior IAEA official said the agency had received no indication of this from Japanese authorities.
"I think at this time we don't have any indication of fuel that is currently melting," IAEA safety official James Lyons said.
A meltdown raises the risk of damage to the reactor vessel and a possible radioactive leak.
Japanese officials have, like Amano, said the thick walls around the radioactive cores of the damaged reactors appeared to be intact.
Japan's crisis has eroded confidence in nuclear energy at a time when many states are considering nuclear energy programmes to reduce reliance on fossil fuels and fight global warming.
Amano said it was too early to say what impact the crisis would have on a revival of nuclear energy use in recent years.
The accident "does not change the fact we need a stable source of energy and we need to mitigate the negative impact of climate change," he said. (Editing by Tim Pearce)