Environmentalists say the possibly catastrophic failure of a Japanese nuclear reactor shows that nuclear power can never be safe.

Two reactors at the Fukushima plant in north-eastern Japan have been damaged by the devastating earthquake which struck nearby on Friday, and at least one is in danger of meltdown.

Radiation has been released into the air after an explosion at one reactor, and although authorities have said it is not intense enough to affect human health, they have ordered evacuations of people living within 20km.

"This proves once and for all that nuclear power cannot ever be safe," Greenpeace campaigns head Steve Campbell said on Monday.

"Japan's nuclear plants were built with the latest technology, specifically to withstand natural disasters, yet we still face potential meltdown," he said in a statement.