The maximum, seventh level on the International Nuclear and Radiological Event Scale (INES) is described as a "major event" and leads to "a major release of radioactive material with widespread health and environmental effects." So far, it has been assigned only to the 1986 Chernobyl disaster.

NISA confirmed the reports, saying that though the plant's total radioactive emission was still only about 10% of that released from Chernobyl, it was still enough to equal the two disasters.

The agency reported on Monday that the plant has been releasing 10,000 terabecquerels of radioactivity per hour during an unidentified period, which is an official benchmark for a category seven event.

The Fukushima disaster was earlier classed as a category five event, or an "accident with wider consequences," which leads to a "limited release of radioactive material."

Japan's Fukushima Daiichi nuclear power plant was seriously damaged by a powerful earthquake and tsunami that hit Japan on March 11. Fukushima's operator has since been struggling to stop radioactive leaks from the plant's crippled reactors.

Chief Cabinet Secretary Yukio Edano said on Monday residents in areas between 20 and 30 kilometers around Fukushima would be advised to evacuate due to accumulated radiation exposure levels.

Despite growing concern of radioactive fallout on the Russian territory, the Russian emergencies ministry said that radiation levels in Russia's Far East remained within the norm and ranged from 11 to 16 micro roentgen per hour on Tuesday.