Ryohei Shiomi, a spokesman for the nuclear safety agency, said 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.
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 kilometres, 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.
Key developments
ATTEMPTS TO COOL REACTORS: Military fire trucks spray seawater for a second day on the stricken Fukushima Dai-ichi nuclear complex in a desperate attempt to prevent its fuel from overheating and spewing dangerous radiation. A U.S. military fire truck joins six Japanese vehicles, but is apparently driven by Japanese workers. Japanese air force says some water appears to have reached its target.
IAEA CALLS ACCIDENT "EXTREMELY SERIOUS." The head of the U.N.'s International Atomic Energy Agency says authorities are "racing against the clock" to cool the complex and calls the accident "extremely serious." He also complains that Japan has not provided adequate information.
NEW POWER LINE NOT YET COMPLETE: The nuclear plant's operator, Tokyo Electric Power, has not finished laying a new power line to the plant in an attempt to restore cooling systems. It remains unclear whether the cooling systems will still function, and officials warned that sparks from electricity could trigger new explosions.
MOMENT OF SILENCE: Tsunami survivors observe a minute of silence at the one-week mark since the quake, which struck at 2:46 p.m. Many lowered their heads and pressed their hands together in prayer. The twin disasters have left thousands dead and missing. Hundreds of thousands are staying in schools and other shelters, as supplies of fuel, medicine and other necessities run short.
PM URGES PEOPLE TO UNITE: Prime Minister Naoto Kan calls the disasters a "great test for the Japanese people" and urges them to unite to "rebuild Japan from scratch."
IMPACT ON ECONOMY: The yen backs away from historic highs and Japanese shares rise after the Group of Seven major industrialized nations promises coordinated intervention in currency markets to support recovery from the disaster. The G-7 pledge comes a day after the yen soared to an all-time high against the dollar, possibly threatening Japanese exports. Japanese automakers, meanwhile, seek alternative parts suppliers to replace those knocked out by the earthquake, which forced most of the country's car production to a halt.
