By Kiyoshi Takenaka and Yoko Kubota
TOKYO (Reuters) - Japan appeared resigned on Monday to a long fight to contain the world's worst atomic crisis in 25 years after high radiation levels complicated work at its crippled nuclear plant.
Engineers have been battling to control the six-reactor Fukushima complex since it was damaged by a March 11 earthquake and tsunami that also left more than 27,000 people dead or missing across Japan's devastated north east.
Radiation at the plant has soared in recent days: latest readings at the weekend showed contamination 100,000 times normal in water at reactor No. 2 and 1,850 times normal in the nearby sea.
Those were the most alarming levels since the crisis began, experts said.
"It's very worrying ... there is something seriously wrong (at No. 2)," said Rianne Teule, a nuclear expert for environmental group Greenpeace based in South Africa.
Under-pressure plant operator Tokyo Electric Power Co. (TEPCO) conceded what experts have long been saying: that Japan now faces a protracted and uncertain operation to contain overheating fuel rods and prevent a meltdown.
"Regrettably, we don't have a concrete schedule at the moment to enable us to say in how many months or years (the crisis will be over)," TEPCO vice-president Sakae Muto said in the latest of round-the-clock briefings the company holds.
Though experts said radiation in the Pacific waters will quickly dissipate, the levels at the site are clearly dangerous, and the 450 or so engineers there have won admiration and sympathy around the world for their bravery and sense of duty.
The nuclear crisis is an especially sensitive subject for Japanese given they are the only nation to have been hit by atomic bombs, at Hiroshima and Nagasaki in 1945.
Last week, three workers at Fukushima were hurt with radiation burns to their legs after water seeped over their boots, and on Sunday engineers had to abandon reactor No. 2 after the new reading.
Further afield, beyond the evacuation zone around Fukushima, there has been plenty of evidence of radiation -- from tap water in Tokyo 240 km (150 miles) south of the nuclear facility to particles found as far away as Iceland.
Japanese officials and international nuclear experts have generally said the levels away from the plant are not dangerous for humans, who anyway face comparable radiation doses on a daily basis from natural substances, X-rays or plane flights.
A clutch of nations including the United States have banned imports of food from the areas closest to the Fukushima plant.
Two of the plant's six reactors are now seen as safe but the other four are volatile, occasionally emitting steam and smoke.
TEPCO officials indicated the highly contaminated water is probably coming from inside the reactors rather than from pools of spent fuel rods outside.
Experts are anxious to find out whether the reactor core is broken and leaking, as that could lead to a meltdown.
One long-term solution may be to entomb the Fukushima reactors in sand and concrete as happened at Chernobyl, Ukraine, after the 1986 disaster that was the world's worst. But there is too much heat for that to be considered yet, experts say.
Yukiya Amano, head of U.N. nuclear watchdog the International Atomic Energy Agency, said the crisis may drag for weeks or months. "This is a very serious accident by all standards," he told the New York Times. "It is not yet over."
The Japan crisis has prompted a reassessment of nuclear power across the world. It had its most direct political impact yet in foreign politics in Germany at the weekend.
Chancellor Angela Merkel's Christian Democrats appeared set to lose control of Germany's most prosperous state, Baden-Wuerttemberg, with voters punishing the party over a U-turn on nuclear power after the Japan crisis.
Away from the plant, radioactivity in Japan's air remained in normal ranges.
In downtown Tokyo, a Reuters reading on Sunday afternoon showed ambient radiation of 0.16 microsieverts per hour, below the global average of naturally occurring background radiation of 0.17-0.39 microsieverts per hour.
The nuclear crisis has compounded Japan's agony after the magnitude 9.0 quake and massive tsunami devastated its north east coast, turning whole towns into apocalyptic-looking landscapes of mud and debris.
The latest death toll was 10,804 people, with 16,244 still missing 17 days after the disasters struck.
About a quarter of a million people are living in shelters.
Damage could top $300 billion, making it the world's costliest natural disaster. Though the disaster will affect Japan's economic growth in the short-term, analysts say that will be quickly compensated by the stimulus of reconstruction.
Facing undoubtedly their most testing moment since World War II, Japanese have reacted with remarkable resilience and unity.
Aid has poured into the disaster zone, refugees have organized themselves meticulously in evacuation centers, and rescuers have not given up the painful but methodical search for bodies and momentos like photographs among the wreckage. (Additional reporting by Chizu Nomiyama, Elaine Lies and Shinichi Saoshiro in Tokyo, Gerard Wynn in London and Alister Doyle in Oslo; Writing by Andrew Cawthorne; editing by Elizabeth Piper)