Japanese authorities evacuated workers on Sunday from a reactor building they were working in after radiation in water at the crippled nuclear power plant reached potentially lethal levels, the plant’s operator said.

Tokyo Electric Power Co said radiation in the water of the No. 2 reactor at the Fukushima Daiichi plant was measured at more than 1,000 millisieverts an hour. That compares with a national safety standard of 250 millisieverts over a year. The US Environmental Protection Agency says a dose of 1,000 millisieverts is enough to cause hemorrhaging.

Japanese nuclear regulators said the water contained 10 million times the amount of radioactive iodine than is normal in the reactor, but noted the substance had a half life of less than an hour, meaning it would disappear within a day.

A Tokyo Electric official said workers were evacuated from the No. 2 reactor’s turbine housing unit to prevent them from being exposed to harmful doses of radiation. They had been trying to pump radioactive water out of the power station after it was found in buildings housing three of the six reactors.

Tokyo Electric engineers have struggled the past two weeks to prevent a catastrophic meltdown at Fukushima Daiichi, after an unprecedented earthquake and tsunami knocked out the backup power system needed to cool the reactors.

The work has had to be suspended several times due to explosions and spiking radiation levels inside the reactors, in a crisis that has become the worst nuclear emergency since Chernobyl a quarter-century ago.

On Thursday, three workers were taken to hospital from reactor No. 3 after stepping in water with radiation levels 10,000 times higher than usually found in a reactor.

The latest radiation scare was confined to inside the reactor. Radiation levels in the air beyond the evacuation zone around the plant and in Tokyo have been in normal ranges.

Yukiya Amano, the director general of the International Atomic Energy Agency (IAEA), cautioned that the nuclear emergency could go on for weeks, if not months more.

"This is a very serious accident by all standards," he told the New York Times. "And it is not yet over."

Radiation levels in the sea off the Fukushima Daiichi plant rose on Sunday to 1,850 times normal just over two weeks after the disaster struck, from 1,250 on Saturday, Japan’s Nuclear and Industrial Safety Agency said.

"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 agency official.

Several countries have banned produce and milk from Japan’s nuclear crisis zone and are monitoring Japanese seacooking over fears of radioactive contamination.

Overshadowing relief effort

The crisis at the plant, 240 km (150 miles) north of Tokyo, has overshadowed a relief and recovery effort from the magnitude 9.0 quake and the huge tsunami it triggered on March 11 that left more than 27,100 people dead or missing in northeast Japan.

The Japanese government estimated last week the material damage from the catastrophe could top $300 billion, making it the world’s costliest natural disaster.

In addition, power cuts have disrupted production while the drawn-out battle to prevent a meltdown at the 40-year-old plant has hurt consumer confidence and spread contamination fears well beyond Japan.

Amano, a former Japanese diplomat who made a trip to Japan after the quake, said authorities were still unsure about whether the plant’s reactor cores and spent fuel were covered with the water needed to cool them.

He told the newspaper he saw a few "positive signs" with the restoration of some electric power to the plant, adding: "More efforts should be done to put an end to the accident."

UN Secretary-General Ban Ki-moon said it was time to reassess the international atomic safety regime.

Japan’s nuclear crisis also looks set to claim its first, and unlikely, political casualty. In far away Germany, Chancellor Angela Merkel’s party faces a defeat in a key state on Sunday, largely because of her policy U-turns on nuclear power.

Not worsening

A Tokyo Electric official told a news conference on Saturday experts were still trying to figure out where to put the contaminated water they’re trying to pump out of the reactors.

They also are not sure where the radiation is leaking from – whether it’s from the spent fuel rod pools or elsewhere in the reactors.

"That’s the problem they have right now, is trying to figure out where this comes from," said Murray Jennex, associate professor at San Diego State University.

"You let (radioactive) stuff accumulate because you don’t have a place to put it. It stays down in the bottom of the plant. If nothing happens, when it comes time to shut it down you clean it up and take care of it. But if something like this happens, that stuff now becomes loose sometimes."

Two of the Fukushima Daiichi reactors are now seen as safe but the other four are volatile, occasionally emitting steam and smoke.

"We are preventing the situation from worsening," Chief Cabinet Secretary Yukio Edano told a news conference on Saturday. "We’ve restored power and pumped in fresh water, and are making basic steps toward improvement, but there is still no room for complacency."

At Chernobyl in Ukraine, the worst nuclear accident in the world, it took weeks to "stabilize" what remained of the reactor that exploded and months to clean up radioactive materials and cover the site with a concrete and steel sarcophagus.

In Tokyo, a metropolis of 13 million, a Reuters reading on Sunday morning showed ambient radiation of 0.06 microsieverts per hour, well within the global average of naturally occurring background radiation of 0.17-0.39 microsieverts per hour, a range given by the World Nuclear Association.