TOKYO - The amount of radioactive cesium that has leaked from the tsunami-hit Fukushima Dai-ichi nuclear plant is about equal to 168 of the atomic bombs dropped on Hiroshima at the end of World War II, Japan's nuclear agency said Friday.

The Nuclear and Industrial Safety Agency supplied the estimate at a parliamentary panel's request. But, it noted, a simple comparison between an instantaneous bomb blast and a long-term leak is impossible and the results could be "irrelevant."

The report provided estimates of each of the 16 isotopes released from the atomic bomb and 31 of those detected at the Fukushima plant but did not give a total. The agency said the radiation that has leaked from Fukushima was about one-sixth of what the Chernobyl nuclear plant disaster released in 1986.

The March 11 earthquake and tsunami in northeastern Japan led three reactor cores at Fukushima Dai-ichi to melt. Several blasts and fires also sent massive amounts of radiation into the environment.

The report said the damaged plant had released 15,000 terabecquerels of cesium-137, which lingers for decades and could cause cancer, compared with the 89 terabecquerels released by the U.S. uranium bomb.

The "Little Boy" bomb dropped on Aug. 6, 1945, destroyed most of Hiroshima and killed as many as 140,000 people. A second atomic bomb three days later dropped on Nagasaki killed tens of thousands more, prompting Japan to surrender, ending WWII.

The Hiroshima bomb claimed most of its victims in the intense heat wave and neutron rays from a midair nuclear explosion and the highly radioactive fallout. No one has died from radiation leaks from the Fukushima plant, where an explosion from hydrogen buildup damaged reactor buildings.

About 100,000 people evacuated their homes because of radiation threats from the Fukushima plant.

On Friday, about 30 residents from the immediate neighborhood of the plant were allowed to briefly return home to get clothes and other necessities. But officials have said the area may stay off-limits for years.

Plant operator Tokyo Electric Power Co. and the government aim to bring the reactors to stable cold shutdown by early January. The government is working to decontaminate areas outside the 12-mile restricted zone.