Japan's government estimates the amount of radioactive caesium-137 released by the Fukushima nuclear disaster so far is equal to that of 168 Hiroshima bombs, a news report said Thursday.

Government nuclear experts, however, said the World War II bomb blast and the accidental reactor meltdowns at Fukushima, which has seen ongoing radiation leaks but no deaths so far, were beyond comparison.

The amount of caesium-137 released since the three reactors were crippled by the March 11 quake and tsunami has been estimated at 15,000 tera becquerels, the Tokyo Shimbun reported, quoting a government calculation.

That compares with the 89 tera becquerels released by "Little Boy", the uranium bomb the United States dropped on the western Japanese city in the final days of World War II, the report said.

The estimate was submitted by Prime Minister Naoto Kan's cabinet to a lower house committee on promotion of technology and innovation, the daily said.

The government, however, argued that the comparison was not valid.

While the Hiroshima bomb claimed most of its victims in the intense heatwave of a mid-air nuclear explosion and the highly radioactive fallout from its mushroom cloud, no such nuclear explosions hit Fukushima.

There, the radiation has seeped from molten fuel inside reactors damaged by hydrogen explosions.

"An atomic bomb is designed to enable mass-killing and mass-destruction by causing blast waves and heat rays and releasing neutron radiation," the Tokyo Shimbun daily quoted a government official as saying. "It is not rational to make a simple comparison only based on the amount of isotopes released."

Government officials were not immediately available to confirm the report.

The blinding blast of the Hiroshima bomb and its fallout killed some 140,000 people, either instantly or in the days and weeks that followed as high radiation or horrific burns took their toll.

At Fukushima, Japan declared a 20-kilometre (12 mile) evacuation and no-go zone around the plant after the March 11 quake and tsunami triggered the world's worst nuclear accident since Chernobyl 25 years ago.

A recent government survey showed that some areas within the 20-kilometre zone are contaminated with radiation equivalent to more than 500 millisieverts per year -- 25 times more than the government's annual limit.