TOKYO - Mistaken radiation readings given out by the operator of Japan's crippled nuclear plant were "absolutely unforgivable", the government's chief spokesman said on Monday, as work to prevent a catastrophic meltdown faced fresh hurdles.

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.

Fires, explosions, and radiation leaks have repeatedly forced them to suspend work, including on Sunday when radiation levels spiked to 100,000 times above normal. Tokyo Electric Power Co , the plant operator, had earlier said it was 10 million times the normal.

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"On one hand, I do think the workers at the site are getting quite tired," Chief Cabinet Secretary Yukio Edano told a news conference. "But these radiation tests are being used for making various decisions on safety and therefore these mistakes are absolutely unforgivable."  

A partial meltdown of fuel rods inside the reactor vessel was responsible for the high levels of radiation at reactor No. 2, Edano said.  

"The airborne radiation is mainly contained within the reactor building. We must make sure this water does not seep out into the soil or out to sea," Edano said.  

The spike in radiation levels forced a suspension of work over the weekend at the reactor, with experts warning that Japan faced a long fight to contain the world's most dangerous atomic crisis in 25 years.

"This is far beyond what one nation can handle - it needs to be bumped up to the UN Security Council," said Najmedin Meshkati, of the University of Southern California. "In my humble opinion, this is more important than the Libya no fly zone."

Tokyo Electric has conceded it faces a protracted and uncertain operation to contain the 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.

He also apologised over the mistaken radiation reading.