The concentration of radioactive iodine in seawater near Japan's crippled Fukushima Nuclear Power Plant is currently 4,385 times above the norm, the Kyodo news agency reported on Thursday.
The figure has rapidly increased over 1,000 times since Wednesday, when levels of radioactive iodine-131 isotope in the sea near the plant were 3,355 times higher than normal, RIA Novosti reported.
The nuclear crisis at Fukushima was prompted by the March 11 earthquake and tsunami that killed more than 11,000 people in northeastern Japan. Radiation leaks were reported after fires and blasts, caused by the collapse of the plant's cooling system.
Japan's Chief Cabinet Secretary Yukio Edano said on Wednesday it was time to start thinking about closing down all six reactors at Fukushima, although reactors No. 5 and 6 have been less problematic than the other four and are already in a state of cold shutdown.