Tokyo Electric Power says initial reports of levels 10m times higher than normal in parts of No 2 reactor were inaccurate

Fresh doubt has been cast on the handling of the Fukushima nuclear crisis after officials admitted wildly overstating levels of radiation, prompting an evacuation of the nuclear site damaged by the 11 March earthquake and tsunami.

Tokyo Electric Power (Tepco) said initial reports of a level 10m times higher than normal in parts of the No 2 reactor were inaccurate, although it could not say by how much.

Tepco said at first that the worker who took the measurement, of a pool of water in the reactor's basement turbine building, had fled before taking a second reading. The discovery prompted another evacuation at the site, halting work to pump and store radioactive water that has built up in the turbine buildings of three of the six reactors.

Tepco later said the pool of water had been contaminated but the extremely high reading was a mistake. "The number is not credible," spokesman Takashi Kurita said. "We are very sorry."

However, later reports on Sunday showed contamination 100,000 times normal in water at reactor No 2, and 1,850 times normal in the nearby sea, the most alarming levels since the crisis began.

Evidence of dangerous contamination in reactor No 2 emerged days after three workers were exposed to high levels while repairing the cooling system at the No 3 reactor. Two of the men received suspected beta ray burns after stepping into water. Reports said the workers were due to be discharged from hospital on Monday.

One pump is being used to extract radioactive water, and two more will be taken to the site. The US military is sending barges loaded with 500,000 gallons of fresh water to nearby Onahama Bay.

Early this morning a magnitude 6.5 earthquake rocked north Japan, the latest aftershock, and officials warned it would trigger a 50 cm (two ft) tsunami.

Two of Fukushima's six reactors are safe, having achieved "cool shutdown", but the other four have yet to be brought under control. Japan's nuclear safety agency, Nisa, said the temperature and pressure inside all six reactors had stabilised.

Yukio Edano, the chief government spokesman, said the myriad problems at the plant were no closer to being resolved. "We have restored power and pumped in fresh water, and we are making basic steps towards improvement. But there is still no room for complacency."

Modest progress was made on removing contaminated water and stepping up work to cool the reactors with fresh water, rather than corrosive sea water, over the weekend. But Yukiya Amano, director general of the International Atomic Energy Agency, said the emergency could continue for weeks, or possibly months.

Concern over food safety spread to fishing over the weekend when officials said seawater samples taken 20 miles off the coast of Fukushima contained 1,850 times the normal level of radioactivity.

Nisa said the tainted seawater posed no risk: "Ocean currents will disperse radiation particles and so it will be very diluted by the time it is consumed by fish and seaweed, and even more by the time they are consumed by humans. There is no need to worry about health risks."

US authorities said on Sunday night that low concentrations of radiation in samples of Massachusetts rainwater were probably caused by Fukushima. Nevada, California, Hawaii, Colorado and Washington state have also reported tiny amounts of radiation from the accident but officials said they presented no health risks.