The accident at Japan's Fukushima Daiichi plant now rates six on seven-point international scale of gravity for nuclear accidents, Andre-Claude Lacoste, head of France's Nuclear Safety Authority (ASN), said on Tuesday.

The 1979 accident at Three Mile Island in Pennsylvania rates five on an international scale of zero to seven, while Chernobyl is put at seven, the highest.

Japan's nuclear safety agency has estimated the accident at Fukushima at level four.

"The incident has taken on a completely different dimension compared to yesterday. It is clear that we are at level six," Lacoste told a press conference.

"The order of gravity has changed," he said.