Japan's nuclear safety agency announced on Tuesday that it has raised the severity level of the accident at the crippled Fukushima No.1 nuclear power plant to 7, the worst on an international scale, from the current 5.

The ranking had only been applied to the 1986 Chernobyl accident. According to Japan's Nuclear and Industrial Safety Agency, the amount of radiation emissions released at the Fukushima plant was equivalent to 10 percent of that in the Chernobyl accident.

The International Atomic Energy Agency described an accident ranked 7 as a major one.

Minoru Oogoda, an official with the Japanese agency, said the agency made the decision because the impact of radiation leaks had widely spread to the air, vegetables, tap water and the ocean.

The Japanese authorities have been battling to keep the troubled Fukushima No.1 nuclear plant under control. The plant has been leaking radioactive substances into environment.

The facility was crippled by the devastating March 11 earthquake and ensuing tsunami that struck the Pacific coastal areas of northeastern and eastern Japan.

The National Police Agency said on Monday that the twin disasters had left 13,116 people dead and 14,377 others unaccounted for in Japan by 10:00 a.m. local time.

About 151,000 survivors are staying in 2,300 shelters across the country.

Meanwhile, a series of aftershocks have been occurring after the catastrophic earthquake and tsunami.

On Tuesday morning, an earthquake with a preliminary magnitude of 6.3 jolted Chiba Prefecture, east of Tokyo, and neighboring areas at 8:08 a.m. local time, according to the Japan Meteorological Agency. No tsunami warning was issued after the quake.