Emergency workers struggling to pump contaminated water from Japan's stricken nuclear complex fled from one of the troubled reactors on Sunday after reporting a huge increase in radioactivity -- a spike that officials later apologetically said was inaccurate.

The apology came after employees fled the complex's No 2 reactor when a reading showed radiation levels had reached 10 million times higher than normal in the reactor's cooling system. Officials said they were so high that the worker taking the measurements had withdrawn before taking a second reading. 

On Sunday night, though, plant operators said that while the water was contaminated with radiation, the extremely high reading was a mistake.

"The number is not credible," said Tokyo Electric Power Co spokesman Takashi Kurita. "We are very sorry."

He said officials were taking another sample to get accurate levels, but did not know when the results would be announced.

The situation came as officials acknowledged there was radioactive water in all four of the Fukushima Daiichi complex's most troubled reactors, and as airborne radiation in No 2 reactor measured 1,000 millisieverts per hour -- four times the limit deemed safe by the government, Kurita said.

Officials say they still don't know where the radioactive water is coming from.

Two of the plant's six reactors are now seen as safe but the other four are volatile, occasionally emitting steam and smoke.

Tokyo Electric Power Co engineers have been working around the clock to stabilize the nuclear 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.

Just outside the coastal nuclear plant, radioactivity in seawater tested about 1,250 times higher than normal last week - but that number had climbed to 1,850 times normal by the weekend.

Hidehiko Nishiyama, a nuclear safety official, said the increase was a concern, but also said the area is not a source of seafood and that the contamination posed no immediate threat to human health.

Experts with the International Atomic Energy Agency said the ocean would quickly dilute the worst contamination.

Up to 600 people are working inside the plant in shifts.

The elevated radiation detected on Sunday was confined to the reactor, and radioactivity in the air beyond the evacuation zone remained in normal ranges.

Several countries have banned products and milk from Japan's nuclear crisis zone.