UPDATE 1-French nuclear agency now rates Japan accident at 6
* Situation clearly a catastrophe -- French safety authority
* U.S. think tank says disaster may reach level seven
* Used only once before, for Chernobyl
(adds think-tank on level 7 possible)
PARIS, March 15 (Reuters) - France's ASN nuclear safety authority said on Tuesday the nuclear accident at Tokyo Electric Power Co's (9501.T) Fukushima Daiichi plant could now be classed as level six out of an international scale of one to seven.
On Monday, the ASN had rated the ongoing accident at the plant, located 240 km (150 miles) north of Tokyo, as a five or six.
Level seven has been used only once, for Chernobyl in Ukraine in 1986. The 1979 accident at the Three Mile Island nuclear power plant in the United States was rated a level five.
"We are now in a situation that is different from yesterday's. 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," ASN President Andre-Claude Lacoste told a news conference in Paris on Tuesday.
"We are clearly in a catastrophe," Lacoste added, citing the deterioration of the containment structure at Daiichi 2 as one of the key elements supporting the ASN's more pessimistic assessment.
Two reactors exploded on Tuesday at the Fukushima Daiichi plant after days of frantic efforts to cool them.
Japan, which rated the accident a four on Saturday, is under global scrutiny over its handling of a nuclear crisis triggered by a huge earthquake and tsunami that crippled three reactors and raised fears of an uncontrolled radiation leak.
A U.S.-based think-tank said the situation had "worsened considerably" and that it was now closer to a level 6 event, "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," the Institute for Science and International Security (ISIS) said in a statement.
"A level 7 event would constitute a larger release of radioactive material, and would require further extended countermeasures," it said, adding the international community should step up assistance to Japan. (Reporting by Mathile Cru in Paris and by Sylvia Westall and Fredrik Dahl in Vienna; writing by Marie Maitre; editing by Matthew Jones)