The idea of anchoring an empty tanker near the troubled No. 2 reactor building so that workers can pump several hundred tons of highly radioactive water into its storage facilities, has also been floated by the government and nuclear experts.

Japan's Nuclear and Industrial Safety Agency said Wednesday that seawater near the Fukushima Daiichi nuclear plant's No.1 reactor contained radioactive iodine at 3,355 times the legal limit.

"The (radiation) figures are rising further," said Hidehiko Nishiyama, the agency's deputy director-general. "We are in an unprecedented situation, so we need to think about different strategies, beyond those we normally think about."

But Nishiyama also said, as there was no fishing in the region, the effects on local residents would be minimal.

"Iodine 131 has a half-life of eight days, and even considering its concentration in marine life, it will have deteriorated considerably by the time it reaches people," Nishiyama said at a news conference.

The National Police Agency said Wednesday that the earthquake and tsunami has so far left 11,232 people dead and 16,361 others unaccounted for.

Also in the day, "extremely low levels" of radioactive iodine were detected in the air around more Chinese areas, including Shanghai and Tianjin, but the materials pose no threat to public health.

China's National Nuclear Emergency Coordination Committee said in a statement that low levels of radioactive isotope iodine-131 were detected Wednesday in 18 provinces.

But the committee said the amount of radiation was below one-hundred-thousandth of the average annual exposure level to natural radioactive sources such as rocks, soil, food and the sun, and no protective measures need to be taken.

Meanwhile, South Korea said Wednesday that radioactive materials were found in some imported Japanese food products, albeit at minuscule levels.

Traces of cesium and iodine were detected in 14 Japanese food products out of 244 tested for possible radioactive contamination, but the minute levels of radiation in them pose no human health risks, the Korea Food and Drug Administration said in a press release.

The spread of radiation has raised concerns about the safety of Japan's seafood, even though experts say the low levels suggest radiation won't accumulate in fish at unsafe levels. Trace amounts of radioactive cesium-137 have been found in anchovies as far afield as Chiba, near Tokyo, but at less than 1 percent of acceptable levels.