Even as engineers tried to pump puddles of radioactive water from the power plant 240 km (150 miles) north of Tokyo, the nuclear safety agency said tests on Friday showed radioactive iodine had spiked 1,250 times higher than normal in the seawater just offshore the plant.

Officials said iodine 131 levels in seawater 30 km (19 miles) from the coastal nuclear complex were within acceptable limits established by regulations and the contamination posed little risk to aquatic life.

"Ocean currents will disperse radiation particles and so it will be very diluted by the time it gets consumed by fish and seaweed," said Hidehiko Nishiyama, a senior official from Japan's Nuclear and Industrial Safety Agency.

Despite that reassurance, the disclosure may well heighten international concern over Japanese seafood exports. Several countries have already banned milk and produce from areas around the Fukushima Daiichi plant, while others have been monitoring Japanese seafood.

The prolonged efforts to prevent a catastrophic meltdown at the plant have also intensified concerns around the world about nuclear power. UN Secretary-General Ban Ki-moon said it was time to reassess the international atomic safety regime.

Radioactive water was found in buildings housing three of the six reactors at the crippled plant. On Thursday, three workers sustained burns at reactor No. 3 after being exposed to radiation levels 10,000 times higher than usually found in a reactor.

The crisis at the nuclear plant has overshadowed the massive relief and recovery effort from the magnitude 9.0 quake and the huge tsunami it triggered on March 11 that left more than 27,500 people dead or missing in northeast Japan.

Nishiyama said Japanese agencies were trying to work out ways of "safely bailing out the water so that it does not get out into the environment, and we are making preparations."

He initially said the high radiation reading inside reactor 3, where the workers were injured, could indicate damage to the reactor. He later said it could be from venting operations to release pressure or water leakage from pipes or valves.

"There is no data suggesting a crack," Nishiyama said. Reactor number 3 is the only one of the six that uses a fuel mixture of plutonium and uranium. Plutonium is the most deadly radioactive isotope.