Rescue workers continue their search and relief mission in heavy snow on Wednesday at an industrial area devastated by last week's earthquake and tsunami in Sendai, northern Japan. [Kim Kyung-hoon / Reuters]

TOKYO - Nuclear regulatory agencies from both the United States and the United Nations separately expressed their growing concerns over the threat posed by Japan's nuclear crisis, with one US official saying the damage at one reactor at the Fukushima Daiichi complex was much more serious than Japanese officials had acknowledged.

The US and the Japanese governments also differed for the first time on advising their citizens on what to do near the Japanese nuclear reactors at the Fukushima plant.

The US expanded its evacuation warnings and told Americans to get at least 80 kilometers away. The Japan government previously recommended that people living within 20 km to evacuate and those who are 30 km away to stay indoors.

Yukiya Amano, director-general of the International Atomic Energy Agency, said he planned to fly to Japan on Thursday for a one-day trip to seek first-hand information from Japan, a strong suggestion that the Vienna-based agency is increasingly frustrated with Japan's lack of speedy and detailed information about the crisis.

"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."

Though both the UN and the US took pains not to criticize the Japanese government, the new international scrutiny spoke of a growing divide with the Japanese about how to handle the situation. It has been five days since Japanese engineers attempted to bring four reactors under control after their cooling systems were damaged by an earthquake and tsunami.

But the chairman of the United States Nuclear Regulatory Commission cast doubt on emergency workers' ability to cool overheating reactors, saying radiation levels may give them "lethal doses" of radiation.

"We believe that around the reactor site there are high levels of radiation," said Gregory Jaczko. "It would be very difficult for emergency workers to get near the reactors. The doses they could experience would potentially be lethal doses in a very short period of time."

An official at Japan's Nuclear and Industrial Safety Agency said on Thursday morning local time that the level of radiation detected at the plant had fallen steadily over the past 12 hours.

Earlier on Wednesday, Japanese workers withdrew briefly from the Fukushima Daiichi Nuclear Power Station because of surging radiation levels. A helicopter failed to drop water on the most troubled reactor.

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

And another fire broke out on Wednesday 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.

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.

At the Fukushima plant, authorities have spent days desperately trying to prevent water, used to cool radioactive cores of the reactors, from evaporating which would lead to overheating and possibly a 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 cool and trap the majority of cesium, iodine and strontium in its water.

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. 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.

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.

The French government said on Wednesday that Japan was losing control of the situation and urged its nationals in Tokyo to leave the country.

French Industry Minister Eric Besson said: "Let's not beat about the bush. They have visibly lost the essential control (of the situation). That is our analysis, in any case, it's not what they are saying."

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.

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.

AP contributed to this story.

Reuters