Engineers have been battling to control the six-reactor Fukushima complex since it was damaged by the March 11 earthquake and tsunami that also left more than 27,000 people dead or missing across devastated northeastern part of Japan.

A magnitude 6.5 earthquake rocked the region on March 28, 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 March 27 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.

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.