A U.S. think-tank has warned the crisis at Japan's earthquake-damaged Fukushima Daiichi power plant could reach the worst level for nuclear accidents.

The Washington, D.C.-based Institute for Science and International Security issued on Tuesday (March 15) the following statement:

ISIS assesses that the situation at the Fukushima Daiichi nuclear plant has worsened considerably. The explosion in the Unit 2 reactor, the third so far, and the fire in the spent fuel pond in the reactor building for Unit 41 means that this accident can no longer be viewed as a level 4 on the International Nuclear and Radiological Events (INES) scale that ranks events from 1 to 7. A level 4 incident involves only local radiological consequences. This event is now closer to a level 6, and it may unfortunately reach a level 7.

A level 6 event means that consequences are broader and countermeasures are needed to deal with the radioactive contamination. A level 7 event would constitute a larger release of radioactive material, and would require further extended countermeasures. The international community should increase assistance to Japan to both contain the emergency at the reactors and to address the wider contamination. We need to find a solution together.

The 1979 partial core meltdown at Three Mile Island in Pennsylvania is classified as a level five event (accident with wider consequences).

1986's Chernobyl disaster, the worst nuclear accident in history, is the only event that has ever been rated level seven (major accident).

France's nuclear safety authority has said the Fukushima crisis is worse than the nuclear accident at Three Mile Island but less severe than Chernobyl.

"It is very clear that we are at a level six, which is an intermediate level between what happened at Three Mile Island and Chernobyl," Autorité de sí»reté nucléaire president Andre-Claude Lacoste told a news conference in Paris on Tuesday, according to Reuters.

"We are clearly in a catastrophe," Lacoste added.

The latest update today (March 16) from the International Atomic Energy Agency states: "Japanese authorities have reported concerns about the condition of the spent nuclear fuel pool at Fukushima Daiichi Unit 3 and Unit 4. Japanese Defense Minister Toshimi Kitazawa announced Wednesday that Special Defence Forces helicopters planned to drop water onto Unit 3, and officials are also preparing to spray water into Unit 4 from ground positions, and possibly later into Unit 3. Some debris on the ground from the 14 March explosion at Unit 3 may need to be removed before the spraying can begin."

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