Radiation levels 15,000 times above normal have been recorded 2km from the site of the nuclear disaster that has engulfed Japan.
"As of late Thursday night, 3.1 millisieverts of neutrons per hour, or about 15,000 times the normal level of radiation, was detected 2km from the accident site," said an official from the Ibaraki Prefecture 70 miles north-east of Tokyo, where the accident occurred.
The levels are too high to allow safety experts to approach the uranium processing plant. "It's not a situation where you can get close to the actual site," said the official said. "What we are trying to do now is come up with measures to contain or extract the radiation from around it."
The accident at the Tokaimura uranium processing plant happened yesterday morning and is the worst in Japan's history. Residents within a 10km (6-mile) danger zone around the site were told to stay inside as officials believed the uranium reached critical level - at which a self-sustaining nuclear reaction starts.
"There is a strong possibility that abnormal reactions are continuing inside even now," Chief Cabinet Secretary Mr Hiromu Nonaka said after the unexplained accident. "The situation is one our country has never experienced," he told a news conference after an emergency government meeting called by the Prime Minister, Mr Keizo Obuchi, which set up a special task force.
Police sealed off a 200-metre area around the facility in Tokaimura, 120km north-east of Tokyo, where about 150 people were evacuated as radiation levels rose, they said.
"Just in case, we have been telling residents through the public address system to stay indoors and not to go outside," said a spokeswoman for the town. "If anyone goes out they should wipe off the rain," she said, as drizzle came down near the site of the accident. Rain continued throughout much of the night, washing nuclear contamination out of the atmosphere and into the ground.
Farmers within the danger zone were ordered by the district government not to harvest crops until their safety had been confirmed. Traffic was barred for a 3km radius around the accident site, and the zone was like a ghost town, with no cars or people, and local train services were stopped.
The extent of the accident was still unclear as it was impossible to enter the plant, one government official said, warning of the danger of an explosion.
A helicopter flew three nuclear workers to hospital after the accident at a "conversion experiment building" of the plant, which processes highly enriched uranium into an oxidised powder form for burning at nuclear power plants. "They were exposed to a high level of radiation," said Dr Hirohiko Tsujii, head of a medical team looking after the injured trio at the National Institute of Radiological Sciences, east of Tokyo.
Two of the workers were carried into the institute on stretchers by medical staff wearing masks and white anti-radiation suits before being rushed into a sterile isolation ward. Their white blood cell counts were sharply up and immune systems were weak, the doctor said.
"They show very strong radiation poisoning symptoms, including diarrhoea," he told a news conference. The two most seriously injured were "barely conscious. They respond to questions but their blood pressure is low," said Dr Tsujii. The third injured man walked into hospital and was treated in a normal ward.
Up to 32 people were exposed to radiation including residents and other workers at the plant. All staff at the plant were later evacuated for safety reasons, operators said.
The head of JCO's Tokyo office, Mr Makoto Ujihara, said the injured plant workers told other staff at the plant "they saw blue flame rising from the fuel" and complained of nausea.
"We are still trying to find what exactly happened but we believe the uranium reached the critical point," Mr Ujihara said.
Five construction workers evacuated from near the facility were also suspected of being lightly exposed to radiation, said a local government official.
The accident in Tokaimura was near another complex where a plutonium reprocessing plant caught fire in March 1997 and exposed 37 people to radiation in what was Japan's previous worst nuclear accident.