Japan's nuclear crisis appeared to be spinning out of control on Wednesday after workers withdrew briefly from a stricken power plant because of surging radiation levels and a helicopter failed to drop water on the most troubled reactor.　

In a sign of desperation, the police will try to cool spent nuclear fuel at one of the facility's reactors with water cannon.

Earlier on Wednesday another fire broke out at the earthquake-crippled nuclear plant, which has sent low levels of radiation wafting into Tokyo, triggering fear in the capital and international alarm.

Workers were trying to clear debris to build a road so fire trucks could reach reactor No 4 at the Fukushima Daiichi complex, 240 km north of Tokyo. Flames were no longer visible at the building housing the reactor.

High radiation levels prevented a helicopter from flying to the site to drop water into the No 3 reactor to try to cool its fuel rods. Its roof was damaged by an earlier explosion and white steam-like clouds drifted up, probably emitting the burst of radiation that led to the workers' withdrawal.

The plant operator described No 3 as the "priority". That reactor is the only one at Daiichi that uses plutonium in its fuel mix.

According to US government research, plutonium is very toxic to humans and once absorbed in the bloodstream can linger for years in bone marrow or liver and can lead to cancer.

The situation at No 4 reactor, where the fire broke out, was "not so good", the plant operator added, while water was being poured into reactors 5 and 6, indicating the entire six-reactor facility was now at risk of overheating.

Nuclear experts said the solutions being proposed to quell radiation leaks at the complex were last-ditch efforts to stem what could well be remembered as one of the world's worst industrial disasters.

"This is a slow-moving nightmare," said Dr Thomas Neff, a physicist and uranium-industry analyst at the Massachusetts Institute of Technology.

In a rare address to the nation, Emperor Akihito expressed condolences and urged Japan not to give up.

He said he was deeply worried by the country's nuclear crisis that was "unprecedented in scale".

"I hope from the bottom of my heart that the people will, hand in hand, treat each other with compassion and overcome these difficult times," the emperor said.