TOKYO (REUTERS) - Japan will raise the severity rating of a recent toxic water leak at the destroyed Fukushima nuclear plant to level 3, or "serious incident", on an international scale for radiological releases, underlining the deepening sense of crisis at the site.

Contaminated water with dangerously high levels of radiation is leaking from a storage tank at Fukushima, the plant's operator said on Tuesday, the most serious setback to date for the clean up of the worst nuclear accident since Chernobyl.

Japan's Nuclear Regulation Authority (NRA) assessed that the leak merits a level 3 rating, classifying it as a "serious incident" on the INES scale, according to a document posted on the agency's website on Wednesday.

The leak had previously been assigned a level 1 "anomaly"rating, but the increase to level 3 is scheduled for formal adoption by the authority's commissioners later on on Wednesday after a meeting that is currently under way, a spokesman for the agency told Reuters by phone.

"Judging from the amount and the density of the radiation in the contaminated water that leaked ... a level 3 assessment is appropriate," according to the document, which is one of several discussion points at the regular weekly meeting of NRA commissioners being held on Wednesday.

This marks the first time Japan has issued an INES rating for Fukushima since the meltdowns in March 2011, following a massive earthquake and tsunami.

Fukushima was assigned the highest rating of 7 after it was hit by explosions in the wake of a loss of power and cooling.

Each one-step increase on the INES scale represents a 10 times increase in severity, according to a factsheet on the website of the International Atomic Energy Agency.