WRAPUP 3 - Disaster-hit Japan faces protracted nuclear crisis

* Battle to control Fukushima plant seen far from over

* Japan crisis helps tip Germany poll against Merkel

* More than 27,000 dead or missing from quake and tsunami

* Magnitude 6.5 quake in north Japan triggers small tsunami

* Low-level radiation found in Massachusetts rainwater (Adds Massachusetts rainwater contaminated)

By Kiyoshi Takenaka and Yoko Kubota

TOKYO, March 28 (Reuters) - Japan appeared resigned on Monday to a long fight to contain the world's most dangerous 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 northeast.

A magnitude 6.5 earthquake rocked the region on Monday, the latest in a series of aftershocks, and officials warned it would trigger a 50-cm (two feet) tsunami wave.

Radiation at the nuclear plant has soared in recent days. Latest readings on Sunday 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.

"I think maybe the situation is much more serious than we were led to believe," said one expert, Najmedin Meshkati, of the University of Southern California, adding it may take weeks to stabilise the situation and the United Nations should step in.

"This is far beyond what one nation can handle - it needs to be bumped up to the U.N. Security Council. In my humble opinion, this is more important than the Libya no fly zone."

Plant operator Tokyo Electric Power Co. has conceded it faces a protracted and uncertain operation to contain overheating fuel rods and avert 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, two workers at Fukushima were injured with radiation burns to their legs after water seeped over their shoes, 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.

In the latest find, traces of radioactive iodine turned up in rainwater samples in Massachusetts in the United States, but health officials there said they posed no threat.

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.

CHERNOBYL ECHOES

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

TEPCO officials indicated the 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 cores are 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.

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 lost control of Germany's most prosperous state, Baden-Wuerttemberg, as anti-nuclear sentiment benefited her opponents in a regional vote.

Japanese Prime Minister Naoto Kan has kept a low profile during the crisis, but may face awkward questions after Kyodo news agency said his visit to the region the day after the disaster delayed TEPCO'S response to the unfolding situation.

"The process to release the steam was delayed due to the premier's visit," because the power company feared Kan could be exposed to radiation, it quoted an unnamed government source as saying.

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 disaster. About a quarter of a million people are living in shelters.

Damage could top $300 billion, making it the world's costliest natural disaster. (Additional reporting by Chizu Nomiyama, Elaine Lies and Shinichi Saoshiro in Tokyo, Gerard Wynn in London and Alister Doyle in Oslo, Scott DiSavino in New York, Christiaan Hetzner in Stuttgart; Writing by Andrew Cawthorne; editing by Bill Tarrant)