The level of radiation in seawater near Japan's Fukushima Dai-ichi nuclear facility has risen to more than 3,300 times what is normal.
While Japanese officials say the development does not pose an immediate threat to human health, they say it is a "concern."
The news came amid reports that Masataka Shimizu, president of the nuclear plant's operator TEPCO, has been hospitalized for dizziness and high blood pressure.
Shimizu's illness comes at an inopportune time for TEPCO. The company has been facing growing criticism over its failure to stop the radiation leaks. Since the March 11 earthquake and subsequent tsunami, TEPCO's shares have plummeted nearly 80 percent.
"We must do everything we can to end this situation as soon as possible for the sake of everyone who has been affected," said Yuhei Sato, governor of Fukushima prefecture. "I am extremely disappointed and saddened by the suggestion that this might drag out longer."
Meanwhile, radiation fears in the U.S. have led to stepped up checks of passengers and cargo coming to Southern California. Almost 3,000 people arrive daily from Japan, and imports top $30 billion.
However, the commissioner in charge of U.S. Customs and Border Protection said that, so far, no dangerous levels of radiation have been detected.
"To date, we've seen nothing to reflect a dangerous situation for the American people here in Los Angeles or across the country," U.S. Customs and Border Protection Commissioner Alan Bersin said Tuesday.