TOKYO -- Radioactive material released into the sea in the Fukushima nuclear power plant crisis is more than triple the amount estimated by plant operator Tokyo Electric Power Co, Japanese researchers say.

Japan's biggest utility estimated around 4,720 trillion becquerels of cesium-137 and iodine-131 was released into the Pacific Ocean between March 21 and April 30, but researchers at the Japan Atomic Energy Agency (JAEA) put the amount 15,000 trillion becquerels, or terabecquerels.

Government regulations ban shipment of foodstuff containing over 500 becquerels of radioactive material per kg.

Takuya Kobayashi, a researcher at the agency, said on Friday the difference in figures was probably because his team measured airborne radioactive material that fell into the ocean in addition to material from contaminated water that leaked from the plant.

He believed Tepco excluded radiation that originally came from airborne material. The report does not include cesium-134 as the research group initially lacked resources to measure it, meaning the amount of estimated radioactive material will increase with further calculations.

The March 11 earthquake and tsunami knocked out reactor cooling systems at Fukushima Daiichi, 240 km (150 miles) north of Tokyo, triggering meltdowns and radiation leaks.

Huge amounts of contaminated water accumulated during efforts to cool the reactors, with much of it reaching the sea, and radiation has been found in fish, seaweed and other seafood.

Tepco edged closer this week to its near-term goal of bringing the reactors to a state of cold shutdown by January, with the temperature at the second of three damaged units falling below boiling point.