TOKYO (Reuters) - 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.

Early in the day another fire broke out at the earthquake-crippled facility, which has sent low levels of radiation wafting into Tokyo in the past 24 hours, triggering fear in the capital and international alarm.

Japan's government said radiation levels outside the plant's gates were stable but, in a sign of being overwhelmed, appealed to private companies to help deliver supplies to tens of thousands of people evacuated from around the complex.

"People would not be in immediate danger if they went outside with these levels. I want people to understand this," Chief Cabinet Secretary Yukio Edano told a televised news conference, referring to people living outside a 30 km (18 miles) exclusion zone. Some 140,000 people inside the zone have been told to stay indoors.

Workers were trying to clear debris to build a road so fire trucks could reach reactor No. 4 at the Daiichi complex in Fukushima, 240 km (150 miles) 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 -- whose roof was damaged by an earlier explosion and where steam was seen rising earlier in the day -- to try to cool its fuel rods.

The plant operator described No. 3 as the "priority." No more information was available, but that reactor is the only one at Daiichi which uses plutonium in its fuel mix.

According to U.S. 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 No. 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.

Japanese Emperor Akihito, delivering a rare video message to his people, said he was deeply worried by the country's nuclear crisis which 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.

Panic over the economic impact of last Friday's massive earthquake and tsunami knocked $620 billion off Japan's stock market over the first two days of this week, but the Nikkei index rebounded on Wednesday to end up 5.68 percent.

Nevertheless, estimates of losses to Japanese output from damage to buildings, production and consumer activity ranged from between 10 and 16 trillion yen ($125-$200 billion), up to one-and-a-half times the economic losses from the devastating 1995 Kobe earthquake.

Damage to Japan's manufacturing base and infrastructure is also threatening significant disruption to the global supply chain, particularly in the technology and auto sectors.

Scores of flights to Japan have been halted or rerouted and air travelers are avoiding Tokyo for fear of radiation. On Wednesday, both France and Australia urged their nationals in Japan to leave the country as authorities grappled with the world's most serious nuclear accident since the Chernobyl disaster in Ukraine in 1986.

The plight of hundreds of thousands left homeless by the quake and devastating tsunami that followed worsened overnight following a cold snap that brought snow to some of the worst-affected areas.

While the death toll stands at around 4,000, more than 7,000 are listed as missing and the figure is expected to rise.

At the Fukushima plant, authorities have spent days desperately trying to prevent water which is designed to cool the radioactive cores of the reactors from evaporating, which would lead to overheating and possibly a dangerous meltdown.

Until the heightened alarm about No.3 reactor, concern had centered on damage to a part of the No.4 reactor building where spent rods were being stored in pools of water, and also to part of the No.2 reactor that helps to cool and trap the majority of cesium, iodine and strontium in its water.

Japanese officials said they were talking to the U.S. military about possible help at the plant.

Concern has mounted that the skeleton crews dealing with the crisis might not be big enough or were exhausted after working for days since the earthquake damaged the facility. Authorities withdrew 750 workers on Tuesday, leaving only 50.

All those remaining were pulled out for almost an hour on Wednesday because radiation levels were too high, but they were later allowed to return.

Arnie Gundersen, a 39-year veteran of the nuclear industry, now chief engineer at Fairwinds Associates Inc and who worked on reactor designs similar to Daiichi plant, said 50 or so people could not babysit six nuclear plants.

"That evacuation (of 750 workers) is a sign they may be throwing in the towel," Gundersen said.

RADIATION IN TOKYO NOT A THREAT TO HUMAN HEALTH

In the first hint of international frustration at the pace of updates from Japan, Yukiya Amano, director general of the International Atomic Energy Agency, said he wanted more timely and detailed information.

"We do not have all the details of the information so what we can do is limited," Amano told a news conference in Vienna. "I am trying to further improve the communication."

Several experts said the Japanese authorities were underplaying the severity of the incident, particularly on a scale called INES used to rank nuclear incidents. The Japanese have so far rated the accident a four on a one-to-seven scale, but that rating was issued on Saturday and since then the situation has worsened dramatically.

France's nuclear safety authority ASN said on Tuesday it should be classed as a level-six incident.

Officials in Tokyo said radiation in the capital was 10 times normal at one point but not a threat to human health in the sprawling high-tech city of 13 million people.

Levels in Ibaraki, north of Tokyo, were 300 times normal, Kyodo news agency said, but still well below hazardous levels.

Many Tokyo residents stayed indoors. Public transport and the streets were as deserted as they would be on a public holiday, and many shops and offices were closed.

Winds over the plant were forecast to blow from the northwest during Wednesday, which would take radiation toward the Pacific Ocean.

"WHAT THE HELL IS GOING ON?"

Japanese media have became more critical of Kan's handling of the disaster and criticized the government and plant operator Tokyo Electric Power Co. for their failure to provide enough information on the incident.

"This government is useless," Masako Kitajima, a Tokyo office worker in her 50s, said as radiation levels ticked up in the city.

Kan himself lambasted the operator for taking so long to inform his office about one of the blasts on Tuesday. A Kyodo news agency reporter quoted the prime minister demanding the power company executives: "What the hell is going on?."

Nuclear radiation is an especially sensitive issue for Japanese following the country's worst human catastrophe -- the U.S. atomic bombs dropped on Hiroshima and Nagasaki in 1945.

The full extent of the destruction was slowly becoming clear as rescuers combed through the tsunami-torn region north of Tokyo where officials say at least 10,000 people were killed.

Whole villages and towns have been wiped off the map by the wall of water, triggering an international humanitarian effort of epic proportions.

There have been hundreds of aftershocks and more than two dozen were greater than magnitude 6, the size of the earthquake that severely damaged Christchurch, New Zealand, last month.

About 850,000 households in the north were still without electricity in near-freezing weather, Tohuku Electric Power Co. said, and the government said at least 1.5 million households lack running water. Tens of thousands of people were missing.

(Additional reporting by Nathan Layne, Linda sieg, Risa Maeda, Isabel Reynolds, Dan Sloan and Leika Kihara in Tokyo, Chris Meyers and Kim Kyung-hoon in Sendai, Taiga Uranaka and Ki Joon Kwon in Fukushima, Noel Randewich in San Francisco, and Miyoung Kim in Seoul; Writing by Nick Macfie and Jason Szep; Editing by John Chalmers and Dean Yates)