Japan's nuclear and industrial safety agency on Friday raised the level for the crisis at the Fukushima Daiichi nuclear plant from a 4 to 5 -- putting it on par with the 1979 incident at Pennsylvania's Three Mile Island.
According to the International Nuclear Events Scale, a level 5 equates to the likelihood of a release of radioactive material, several deaths from radiation and severe damage to a reactor core.
Chernobyl, for example, rated a 7 on the scale, while Japan's other nuclear crisis -- a 1999 accident at Tokaimura in which workers died after being exposed to radiation -- was a 4.
In Pennsylvania, a partial meltdown of a reactor core was deemed the worst nuclear accident in U.S. history.
Significant amounts of radiation have been released after the 9.0-magnitude earthquake hit on March 11, followed by a tsunami that swept away cars and houses along its path. The disasters spurred several explosions at the nuclear plant on the northeastern coast of Honshu.
Relatively high, but officially non-hazardous amounts of radiation have been detected in the air and water of Fukushima city, about 80 kilometers (50 miles) from the plant.
But conditions at the Fukushima Daiichi plant itself remain very dangerous.
Radiation levels Thursday hit 20 millisieverts per hour at an annex building where workers have been trying to re-establish electrical power, "the highest registered (at that building) so far," a Tokyo Electric official told reporters.
By comparison, the typical resident of a developed country is naturally exposed to 3 millisieverts per year.