He had said earlier there was no link between the radioactive water leaking inside the plant and the radiation in the sea. On Monday, though, Nishiyama said he suspects radioactive water from the plant is leaking into the ocean.
Closer to the plant, radioactivity in seawater tested about 1,250 times higher than normal last week and climbed to 1,850 times normal over the weekend. Nishiyama said the increase was a concern but the area was not a source of seafood.
It could take weeks to clear out the radioactive water, said Gary Was, a nuclear engineering professor at the University of Michigan.
"Battling the contamination so workers can work there is going to be an ongoing problem," he said.
Japan's nuclear watchdog, the Nuclear Safety Commission, said Monday that its members — government-appointed experts who monitor the atomic industry — believe that the highly radioactive water came from the containment vessel. It did not clearly state that the primary containment vessel, which protects the core, had been breached.
The commission warned that radioactive water was seeping from the plant into soil and seawater, NISA official Kenji Kinjo said.
Edano, the government spokesman, urged residents to stay out of the 12-mile (20-kilometer) evacuation zone around the plant, saying contaminants posed a "big" health risk. He was responding to reports that people had been sneaking back in without government approval.
Meanwhile, a strong earthquake shook the region and prompted a brief tsunami alert early Monday. The quake off the battered coast of Miyagi prefecture in the northeast was measured as a magnitude 6.5, the Japan Meteorological Agency said.No damage or injuries were reported, and TEPCO said the quake would not affect work to stabilize the plant.
Scores of strong earthquakes have rattled Japan over the past two weeks, adding to the sense of unease across Japan, where the final death toll from the March 11 disasters is expected to top 18,000.