The nuclear crisis at the quake-damaged Fukushima Dai-ichi power plant is now rated just two notches below the 1986 Chernobyl disaster in Ukraine after Japanese authorities raised the alert level at the site to five from four (on a seven-point global scale for such incidents.
Fukushima is now on the same level as the 1979 Three Mile Island crisis in the U.S.
The Japanese nuclear agency cited that the situation at Fukushima is an accident with wider consequences.
Similarly, the chief of the International Atomic Energy Agency (IAEA),the UN's nuclear watchdog, said in Tokyo that the struggle to stabilize Fukushima is now 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 Yukiya Amano, the IAEA head and himself a Japanese citizen.
Amano said he would not visit the Fukushima Daiichi site but that four members of his staff of nuclear experts will monitor radiation in Tokyo, before moving onto the northeast where the stricken power plant is located.
The IAEA boss said he will hold an extraordinary meeting on Japan's nuclear plants next Monday.
Amano also urged Japanese Prime Minister Naoto Kan to provide more specific information on the Fukushima nuclear reactors.
We have been receiving information but there is the opinion in the international community that more detailed information is needed, Amano told reporters at the prime minister's residence.
This is a very grave and serious accident.
As Amano expressed his grave concerns, hundreds of nuclear plant workers are battling to restore power at the Fukushima plant, while hoping to cool down the six reactors there.
My eyes well with tears at the thought of the work they are doing, Kazuya Aoki, a safety official at Japan's Nuclear and Industrial Safety Agency, said, according to The Daily Telegraph. We all just want to support them, and help them do a solid job.
Meanwhile, Japanese authorities stated that 6,405 people are confirmed to have died in the earthquake-tsunami tragedy and more than 10,000 are listed as missing.