6.46pm GMT: A nuclear expert speaking to CNN said that "a worst case scenario" was a Chernobyl-style meltdown if power and cooling could not be restored. But he stressed that it was a remote possibility.
6.41pm GMT: More details on the planned release of radioactive vapour to reduce pressure on the damaged Fukushima Daiichi nuclear plant, where Japan's nuclear safety agency said pressure in one of six boiling water reactors had risen to 1.5 times the level considered normal.
The Associated Press reports:
After the quake triggered a power outage, a backup generator also failed and the cooling system was unable to supply water to cool the 460-megawatt No. 1 reactor, though at least one backup cooling system is being used. The reactor core remains hot even after a shutdown.
The agency said plant workers are scrambling to restore cooling water supply at the plant but there is no prospect for immediate success.
Chief Cabinet Secretary Yukio Edano said the 40-year-old plant was not leaking radiation. The plant is in Onahama city, about 170 miles (270 kilometers) northeast of Tokyo.
If the outage in the cooling system persists, eventually radiation could leak out into the environment, and, in the worst case, could cause a reactor meltdown, a nuclear safety agency official said on condition of anonymity, citing sensitivity of the issue.
Another official at the nuclear safety agency, Yuji Kakizaki, said that plant workers were cooling the reactor with a secondary cooling system, which is not as effective as the regular cooling method.
Kakizaki said officials have confirmed that the emergency cooling system the last-ditch cooling measure to prevent the reactor from the meltdown is intact and could kick in if needed.
"That's as a last resort, and we have not reached that stage yet," Kakizaki added.