Highly radioactive water has leaked from a reactor at the Fukushima nuclear complex in Japan.

The plant, 240km north of Tokyo, was damaged in the 11 March earthquake and tsunami that left more than 27,000 people dead or missing across northeast Japan.

Fires, explosions and radiation leaks have repeatedly forced engineers to suspend efforts to stabilise the plant, including yesterday when radiation levels spiked to 100,000 times above normal in water inside reactor No 2.

A partial meltdown of fuel rods inside the reactor vessel was responsible for the high levels of radiation, although Chief Cabinet Secretary Yukio Edano insisted the radiation had mainly been contained in the reactor building.

TEPCO now says radiation above 1,000 millisieverts per hour has been found in water in underground concrete tunnels that extend beyond the reactor.

That is the same as the level discovered on Sunday. The US Environmental Protection Agency says a single dose of 1,000 millisieverts is enough to cause haemorrhaging.

TEPCO officials said the tunnels did not flow into the sea but the possibility of radioactive water seeping into the ground can not be ruled out.

Greenpeace said its experts had confirmed radiation levels of up to ten microsieverts per hour in a village 40km northwest of the plant - it has called for the extension of the 20km evacuation zone.

'It is clearly not safe for people to remain in Iitate, especially children and pregnant women, when it could mean receiving the maximum allowed annual dose of radiation in only a few days,' Greenpeace said in a statement, referring to the village where the radiation reading was taken.

More than 70,000 people have been forced to leave the exclusion zone and another 130,000 people within a zone extending a further 10km from the plant have been told to stay indoors.

On the weekend, the spike in radiation levels forced a suspension of work at the reactor, with experts warning that Japan faces a long fight to contain the nuclear crisis.

Public information

Mistaken radiation readings given out by the operator of Japan's crippled nuclear plant were described as 'absolutely unforgivable'.

Yesterday radiation levels spiked to 100,000 times above normal - but Tokyo Electric Power had reported a figure of ten million times above normal.

'On one hand, I do think the workers at the site are getting quite tired,' Chief Cabinet Secretary Yukio Edano told a news conference.

'But these radiation tests are being used for making various decisions on safety and therefore these mistakes are absolutely unforgivable.'

The spike in radiation levels forced a suspension of work over the weekend at the reactor.

'This is far beyond what one nation can handle - it needs to be bumped up to the UN Security Council,' said Najmedin Meshkati, of the University of Southern California. 'In my humble opinion, this is more important than the Libya no-fly zone.'