TOKYO – 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.

Related readings:  Japan says very high radiation reading at reactor was wrong  Huge jump in radiation inside Japan nuclear plant  Japan crisis far from end: UN nuclear chief  Breach in reactor suspected at Japan nuke plant

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 stabilize 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 UN 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.