Nuclear and health experts said an increase in the crisis level of Fukushima Daiichi nuclear accident from level 5 to level 7 does not mean that the public health risk is any worse or that the disaster resembles Chernobyl in 1986, the worst nuclear power accident in history, which was also a 7.

World Health Organization (WHO) explained that the higher rating was the result of combining the amounts of radiation leaking from three reactors and counting them as a single incident.

"At the moment there is very little public health risk outside the 30-km (evacuation) zone," Gregory Hartl, spokesman for WHO said.

According to Hartl, the Japanese authorities are looking at the cumulative dose at the reactor itself. "Remember there is no one left around the reactor, it has been evacuated," he said.

The International Atomic Energy Agency also said Fukushima disaster and Chernobyl are very different. "Chernobyl had a reactor in power. It was a huge explosion, a power explosion, and then you had a huge graphite fire for a number of days," Deputy Director General Denis Flory said.

Experts said the amount of discharged radioactive materials is approximately 10 percent of the Chernobyl accident while Japan's Nuclear and Industrial Safety Agency (NISA) said the levels for radioactive iodine and cesium that have been spewed into the air, water, and soil around the crippled nuclear plant are in the thousands of trillions of bequerels -- 15 times higher than the threshold for a top-scale event.

China's Nuclear Safety Agency, meanwhile, also downplayed radiation from Japan's leaking Fukushima Daiichi plant as parallel with Chernobyl.

The Chinese Ministry of Environmental Protection, which also acts as their nuclear safety watchdog, said, "Its impact on our country's environment has been small, equivalent to about one percent of the impact of the Chernobyl nuclear accident on our country."

Nonetheless the Ministry said, "There is a need to adopt protective measures as the long-term consequences of the Fukushima accident cannot be ignored."

Over the weekend Japanese authorities had increased the danger zone around the badly damaged nuclear plant from 20 to 30 km radius as a precaution for a possiblity that "things may turn for the worse".