A meltdown threatens at one or more of Fukushima Daiichi's nuclear power reactors in northern Japan, a senior official has told CNN in the USA.
"We see the possibility of a meltdown," said Toshihiro Bannai, director of the international affairs office of the Japanese Nuclear and Industrial Safety Agency.
He was speaking to CNN in a telephone interview from the agency's headquarters in Tokyo.
A meltdown is a catastrophic failure of the reactor core, with a potential for widespread radiation release.
Another official said on 13 March that the Japanese authorities are operating on the presumption that possible meltdowns are in fact under way at two nuclear reactors.
An estimated 170,000 people have been evacuated from the area. 15 people at a nearby hospital have been found to have been exposed to radiation.
Although Dr Bannai said engineers have been unable to get close enough to the core of reactor number 1 to know what is going on, he based his assessment on the fact that they measured radioactive isotopes in the air on the night of 12 March 2011.
"What we have seen is only the slight indication from a monitoring post of cesium and iodine," he suggested, adding that plant officials have injected sea water and boron into the plant in an effort to cool its nuclear fuel and stop any reactions.
He said he believed that it was still possible to stabilise and secure the situation.
A state of emergency has been declared for the Fukushima Daiichi reactor and two others at the same complex, which holds a total of six, he said. Three are in a safe, shut-down state, while "the other two still have some cooling systems, but not enough capacity."
"There are now problems at the number three reactor - the concern is that it is overheating. They're trying to pump sea water through it at the moment. That's an unusual, somewhat innovative solution to the problem. But the fact that they're prepared to consider unusual solutions like that gives you a hint of just how serious the problem is," reports the BBC's Chris Hogg.
Ichiro Fujisaki, Japan's ambassador to the United States, told CNN in Washington that he knew of no evidence of a meltdown.
Government spokesman Yukio Edano said air with some radioactive content was being released to help to cool the reactor.
With acknowledgments to CNN - http://tinyurl.com/4md98qq - and other news agencies