Japan has raised the alert level at the crippled Fukushima Daiichi plant from four to five on a seven-point international scale for nuclear accidents.
The move places the crisis at the Fukushima Daiichi site two levels below Ukraine's 1986 Chernobyl disaster. It also places the crisis on a par with the Three Mile Island nuclear accident in the US in 1979.
The Japanese nuclear agency's decision to raise the alert level to five grades the Fukushima situation as an "accident with wider consequences, according to the BBC.
The head of the International Atomic Energy Agency (IAEA), Yukiya Amano, said the Fukushima crisis was a "race against the clock".
"This is not something that just Japan should deal with, and people of the entire world should co-operate with Japan and the people in the disaster areas," said Mr Amano, a Japanese citizen.
He said he would not visit the Fukushima Daiichi site on his current trip to the country.
His four-member team of nuclear experts would start by monitoring radiation in the capital, he said, before moving to the vicinity of the quake-hit facility, reports Kyodo news agency.