TOKYO — Japanese officials reported a huge jump in radioactivity — levels 10 million times the norm — in water in one reactor unit at a tsunami-damaged nuclear plant Sunday, forcing workers to evacuate and again delaying efforts to control the leaking complex.

The air, meanwhile, measured 1,000 millisieverts per hour — four times the limit of 250 millisieverts deemed safe by the government, Tokyo Electric Power Co. spokesman Takashi Kurita told reporters.

Word of the startling jump in radioactivity in Unit 2 came as TEPCO struggled to pump the contaminated water from four troubled reactor units at the overheated Fukushima Dai-ichi plant, 140 miles (220 kilometers) northeast of Tokyo. The reading was so high that the worker measuring the levels fled before taking a second reading, officials said.

Japan's Nuclear and Industrial Safety Agency had warned Saturday that radioactivity inside the units was rising fast and that extracting the radioactive water was a priority.

The discovery over the last three days of radioactive water in one or more 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.

The magnitude-9 quake off Japan's northeast coast March 11 triggered a tsunami that barreled onshore and disabled the Fukushima plant, complicating a humanitarian disaster that has killed more than 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 was expected to top 18,000.

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

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. Two workers were being treated at a hospital for possible burns sustained from wading into the contaminated water.

Chief Cabinet Secretary Yukio Edano said the exact source of the leak wasn't clear yet but radioactive water is "almost certainly" seeping from a reactor core.

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

With just one pump incapable 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.

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

Edano urged TEPCO to be more transparent after NISA 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.

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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 spur radiation sickness and death in a large population.

Up to 600 people are working inside the plant in shifts. Nuclear safety officials said the time workers spent 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 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.

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