Japan's nuclear safety agency raised the severity rating of the country's nuclear crisis Friday, March 18, from Level 4 to Level 5 on a seven-level international scale, putting it on par with the Three Mile Island accident in Pennsylvania in 1979.

Ryohei Shiomi, a spokesman for the nuclear safety agency, said that the agency raised the rating of the Fukushima Dai-ichi nuclear crisis on the 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, the Associated Press reports.

A partial meltdown at Three Mile Island also was ranked a Level 5. The Chernobyl accident of 1986, which killed at least 31 people with radiation sickness, raised long-term cancer rates, and spewed radiation for hundreds of miles (kilometers), was ranked a Level 7.

France's Nuclear Safety Authority has been saying since Tuesday that the crisis in northeastern Japan should be ranked Level 6 on the scale.