TOKYO (Reuters) - Workers were withdrawn from a reactor building at Japan's earthquake-wrecked nuclear plant on Sunday after potentially lethal levels of radiation were detected in water there, a major setback for the effort to avert a catastrophic meltdown.

The operator of the facility said radiation in the water of the No. 2 reactor was measured at more than 1,000 millisieverts an hour, the highest reading so far in a crisis triggered by a massive earthquake and tsunami on March 11.
That compares with a national safety standard of 250 millisieverts over a year. The U.S. Environmental Protection Agency says a single dose of 1,000 millisieverts is enough to cause hemorrhaging.
Plant operator Tokyo Electric Power Co. later said the extremely high radioactivity readings might have been wrong, adding the levels were being re-checked.
""The situation is serious. They have to pump away this water on the floor, get rid of it to lower the radiation,"" said Robert Finck, radiation protection specialist at the Swedish Radiation Safety Authority, speaking before the operator expressed doubt about the high reading.
""It's virtually impossible to work, you can only be there for a few minutes. It's impossible to say how long it will take before they can gradually take control.""
The Japanese government said the overall situation was unchanged at the plant, 240 km (150 miles) north of Tokyo. Two of the plant's six reactors are now seen as safe but the other four are volatile, occasionally emitting steam and smoke.
""We did expect to run into unforeseen difficulties, and this accumulation of high radioactivity water is one such example,"" Chief Cabinet Secretary Yukio Edano told a news briefing.
Yukiya Amano, director-general of the International Atomic Energy Agency, said the emergency could go on for weeks, if not months. ""This is a very serious accident by all standards,"" he told the New York Times. ""And it is not yet over.""
At Chernobyl in Ukraine a quarter of a century ago, 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.
Experts say there is still too much heat in the reactor cores and spent fuel at the Fukushima plant for a similar last-ditch solution to be considered yet.
Hundreds of Tokyo Electric Power Co. engineers have been working around the clock to stabilize the Fukushima Daiichi plant since the earthquake and tsunami knocked out the back-up power system needed to cool the reactors.
The operation has been suspended several times due to explosions and spiking radiation levels inside the reactors.
Last 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 scare came as engineers were trying to pump radioactive water out of a turbine unit after it was found in buildings housing three of the reactors.
Officials at first said the water in No. 2 was found to contain 10 million times the amount of radioactive iodine that is normal in the reactor, but noted the substance had a half-life of under an hour, meaning it would disappear within a day.
Later they said the element that gave the reading may have been cobalt 56, which has a half life of 77 days, and if this was the case the level of radioactivity would have been far lower.
Radiation levels in the sea off the plant rose on Sunday to 1,850 times normal, 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.
Tokyo radiation levels normal
The elevated radiation detected on Sunday was confined to the reactor, and radioactivity in the air beyond the evacuation zone around the plant remained in normal ranges.
In downtown Tokyo, a Reuters reading on Sunday afternoon showed ambient radiation of 0.16 microsieverts per hour, below the global average of naturally occurring background radiation of 0.17-0.39 microsieverts per hour.
Several countries have banned produce and milk from Japan's nuclear crisis zone and are monitoring Japanese seafood because of fears of radioactive contamination.
Kyodo news agency said Japan would call on World Trade Organization members at a meeting this week not to overreact to the radiation scare and abide by rules that ban import restrictions not based on scientific grounds.
The accident has also triggered concern around the globe about the safety of nuclear power generation. U.N. Secretary-General Ban Ki-moon said it was time to reassess the international atomic safety regime.
The crisis looked set to claim its first, and unlikely, political casualty. In Germany, Chancellor Angela Merkel's party faced defeat in a key state on Sunday, largely because of her U-turn on nuclear power.