TOKYO — Japan remained on "maximum alert" today over the crippled Fukushima nuclear plant as the government revealed high levels of radiation had been detected in the sea near the facility.

TOKYO — Japan remained on "maximum alert" today over the crippled Fukushima nuclear plant as the government revealed high levels of radiation had been detected in the sea near the facility.

The March 11 earthquake and tsunami that struck Japan's northeast coast and left about 28,000 dead or missing also knocked out the plant's reactor cooling systems, leaking radiation into the air and water.

A government spokesman told the Jiji press agency that the level of radioactive iodine found in the sea near the plant was 3,355 times above the legal limit.

Prime Minister Naoto Kan on Tuesday conceded the situation at the plant remained "unpredictable" and pledged his government would "tackle the problem while in a state of maximum alert," he said.

Workers have poured thousands of tons of water onto reactors where fuel rods are thought to have partially melted, and topped up pools for spent fuel rods.

But the water has been accumulating in the basements of turbine rooms connected to three reactors and filled up tunnels, making it too dangerous for workers to go near to repair cooling systems needed to stabilize the plant.

"Continuing the cooling is unavoidable ... We need to prioritize injecting water," government spokesman Yukio Edano said.

If the rods are completely exposed to air, they will rapidly heat up, melt down and spew out far greater plumes of radiation, nuclear experts fear.

Workers have placed sandbags and concrete blocks around the tunnel shafts to contain the water. They have also restored light in the control rooms of reactors one to four.

However, the water out of the No. 2 reactor has measured 1,000 millisieverts per hour — four times the recently-hiked total exposure limit for emergency staff, and high enough to cause radiation sickness such as nausea and vomiting.

The race to save that reactor may already be lost, a nuclear expert cited by the Guardian newspaper said. Richard Lahey, former head of safety research for boiling-water reactors at General Electric when the company installed the units at Fukushima, said evidence suggests that the No. 2 reactor's radioactive core has melted.

"The indications we have, from the reactor to radiation readings and the materials they are seeing, suggest that the core has melted through the bottom of the pressure vessel in unit two, and at least some of it is down on the floor of the drywell," Lahey said, adding that a disaster similar to Chernobyl is unlikely.