Workers at a tsunami-damaged nuclear plant were forced to evacuate after radiation levels 10 million times above the normal rate were reported in a reactor unit Sunday, but the reading was an error, officials say.

Officials quickly adjusted the threat warnings on Sunday, saying the reported measurements were a mistake and that water did not test 10 million times than usual.

Officials said that while the water was still contaminated with radiation, the extremely high reading was inaccurate.

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

According to officials, the workers fled even before radiation experts were able to take a second reading because levels were so high, meaning efforts to control the leaking complex would be further delayed.

Radioactivity in the air also tested above normal limits, measuring 1,000 millisieverts per hour, which is four times the usual level deemed safe said officials from Tokyo Electric Power Co.

The news of the jump in radioactivity came as TEPCO struggled to pump contaminated water from four troubled reactor units at the Fukushima Dai-ichi plant, which sits 220 kilometers northeast of Tokyo.

Japan's Nuclear and Industrial Safety Agency (NISA) said radioactivity inside the units was rising quickly and that extracting the radioactive water was a priority.

The discovery over the last three days of radioactive water in several units at the six-unit complex has been a major setback in the urgent mission to get the plant's crucial cooling system back up and operating more than two weeks after a massive earthquake and tsunami.

Chris Johnson, a freelance journalist reporting from Morioka, told CTV News Channel Sunday morning that although relief efforts have begun, they are struggling.

"They are trying to lift boats that have sunk in the harbour and move them because these boats are blocking the channel. They are trying to get bigger ships in with relief supplies. It's been more than two weeks since the tsunami and they're still just getting started."

Chief Cabinet Secretary Yukio Edano acknowledged emergency workers still needed to figure out the source of the radioactive water, but insisted the situation had stabilized — at least partially.

"We have somewhat prevented the situation from turning worse," Edano told reporters on Sunday. "But the prospects are not improving in a straight line and we've expected twists and turns. The contaminated water is one of them and we'll continue to repair the damage."

Part of the problem now, according to TEPCO, is where to put the contaminated water they manage to remove.

The situation on the ground has left many Japanese people worried for their livelihoods.

Radiation has been found in milk, seawater and a range of vegetables, including broccoli, cauliflower and turnips.

And although the official death toll stood at 10,489 on Sunday, most expect the final death toll to be about 18,000.

Two workers being treated for burns are to be released from hospital on Monday after wading into water contaminated by radiation levels 10,000 times above safe levels, Edano said.

Around the reactor at the coastal Fukushima nuclear plant, radioactivity in seawater tested about 1,250 times higher than normal, although officials said the area is not a source of seafood and that the contamination posed no immediate threat to human health.

However amidst what some see as a deteriorating situation, public support for Japan's prime minister is recovering according to a new poll.

The survey by Kyodo News agency found approval of Prime Minister Naoto Kan and his Cabinet rose to 28.3 per cent after sinking below 20 per cent in February.

The dismal approval of Kan last month led to speculation he could be forced out of office he already is Japan's fifth leader in four years. While the latest figure is still low, it suggests he is making some gains with voters.

About 58 per cent of respondents in the nationwide survey conducted Saturday and Sunday said they approved of the government's handling of the March 11 earthquake and tsunami, but a similar number criticized its handling of the ongoing crisis at a crippled nuclear plant.

Yesterday the Japanese government also criticized TEPCO for not being transparent enough and revealed that the plant operator was aware of high levels of radiation in the air in Unit 3 several days before the two workers suffered burns there. NISA said it warned TEPCO to improve and ensure workers' safety.

The workers who stepped into the contaminated water Thursday were exposed to radiation at levels between 2,000 and 6,000 millisieverts, the IAEA said Sunday. Health authorities say whole-body exposure to more than 3,000 millisieverts in a short period can cause radiation sickness and even death.

Up to 600 people are working inside the plant in shifts. Nuclear safety officials said the time that workers spend inside units is closely monitored.

"It's definitely a severe environment, but the amount of time workers are allowed in there is strictly controlled so that their exposure does not exceed the limit," NISA official Minoru Ogoda said Sunday.

He said the radioactivity found in Unit 2 is mostly from iodine-134, which has a relatively short half-life of 53 minutes, meaning it dissipates quickly.

With files from The Associated Press