An army helicopter is preparing to pour water into a damaged reactor at the Fukushima Daiichi nuclear power plant in northern Japan, where radiation levels have been rising dangerously.
Overheating fuel rods in the plant's reactors, damaged in last week's massive earthquake and tsunami, or by subsequent explosions and fires, have to be cooled to prevent a full-scale meltdown.
Up to now, workers have been using firefighting equipment to pump seawater into the reactors.
The Japanese government says it's prepared to accept help from the United States military to avert a catastrophe.
Workers trying to stabilise the reactors were briefly evacuated earlier on Wednesday because of fluctuating radiation levels but have since returned.
A second reactor fire at the plant appears to be under control. It's the second fire in two days: the first was caused by a hydrogen explosion that blew a hole in the building housing the No 4 reactor.
It was put out, but not before dangerous levels of radiation escaped. Two workers have been missing since the explosion.
Threat level raised
The French atomic safety authority says the Fukushima Daiichi crisis could now be classed as six on an international scale of one to seven. Two days ago the authority rated it as a five or six.
Level seven has been used just once, for the Chernobyl disaster in Ukraine in 1986.
The 1979 accident at Three Mile Island in the United States was rated a five.
American think-tank the Institute for Science and International Security agrees with the French assessment, saying the situation in Japan could even reach level seven.
Surface normality in Tokyo
The BBC reports that, while radiation levels in Tokyo have risen only slightly, citizens are still unnerved.
On the surface, daily life in the capital - 250 kilometres south of the Fukushima plant - continues as normal, with most businesses still open.
But people have been stocking up on food and petrol, there are fewer cars in the streets, and many foreigners are starting to leave.
Some airlines from Asia and Europe have cancelled flights to Tokyo.