Japan's nuclear safety agency has raised the severity rating of the country's nuclear crisis from Level 4 to Level 5 putting it on par with the Three Mile Island accident in the US n 1979.

Ryohei Shiomi, a spokesman for the nuclear safety agency, said the agency raised the rating of the Fukushima Dai-ichi nuclear crisis on the seven-level International Nuclear Event Scale.

The scale defines a Level 4 incident as having local consequences and a Level 5 incident as having wider consequences.

The hallmarks of a Level 5 emergency are severe damage to a reactor core, release of large quantities of radiation with a high probability of "significant" public exposure or several deaths from radiation.

A partial meltdown at Three Mile Island was also ranked a Level 5.

The Chernobyl accident in the Ukraine in 1986, which killed at least 31 people with radiation sickness, raised long-term cancer rates, and spewed radiation for hundreds of miles, was ranked a Level 7.

France's Nuclear Safety Authority has been saying that the crisis in north-eastern Japan should be ranked Level 6 on the scale.