Japanese officials reported a huge jump in radioactivity in water in one reactor unit at a tsunami-damaged nuclear plant on Sunday, but later said the figure they released was inaccurate.

Takashi Kurita, a spokesman for the Tokyo Electric Power Co. had said the readings showed water testing 10 million times higher in radioactivity than normal in the cooling system of the reactor in northeastern Japan.

On Sunday night, though, plant operators said while the water was contaminated with radiation, they insisted they had made a mistake in the measurement.

"The number is not credible. We are very sorry," said Kurita, who did not release new information on the levels in the water.

Earlier, he said radiation in the air measured 1,000 millisieverts per hour — four times the limit deemed safe by the government.

Word of the jump in radioactivity in Unit 2 came as the power company struggled to pump contaminated water from four troubled reactor units at the overheated Fukushima Daiichi plant, 220 kilometres northeast of Tokyo. Officials said the reading so high that the worker measuring the levels fled before taking a second reading.

Teachers wash backpacks found at their devastated elementary school by the bank of a river in Otsuchi, northeastern Japan, on Sunday. Motoki Nakashima/The Yomiuri Shimbun/AP

Japan's Nuclear and Industrial Safety Agency had warned Saturday that 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.

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 Sunday evening. "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."

The magnitude-9 quake off Japan's northeast coast on March 11 triggered a tsunami that barrelled onshore and disabled the Fukushima plant, complicating a humanitarian disaster that has killed well over 10,000 people and left hundreds of thousands homeless.

The official death toll stood at 10,489 on Sunday, with more than 16,620 people missing, police said. The final death toll is expected to top 18,000.

Since the quake and tsunami, nuclear workers have raced to cool the plant as radiation made its way into food, seawater and even tap water supplies as far away as Tokyo.

2 workers treated for burns

Officials said the discovery Thursday of highly radioactive pools of water in Unit 3 led to suspicions that radiation was leaking due to a possible breach in the reactor's core. Two workers are being treated at a hospital for possible burns sustained from wading into the contaminated water. They are to be released from the hospital Monday, Edano said.

Workers are scrambling to remove the contaminated water and find a safe place to store it, TEPCO officials said.

With just one pump capable of handling the large amounts of water, two more will be brought in to help speed up the process, said Hidehiko Nishiyama, a nuclear safety official.

The protracted nuclear crisis has spurred concerns about the safety of food and water in Japan, which is a prime source of seafood for some countries.

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

Tap water in several areas of Japan, including Tokyo, has showed higher-than-normal levels of radiation, prompting officials to distribute bottled water to families with infants.

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