Radiation levels at the Tokaimura nuclear fuel-processing plant in north-east Japan are 15,000 times higher than normal.

The authorities have warned thousands of residents near the site of the accident to stay indoors and to wash off any rain that falls on them.

"There is a strong possibility that abnormal reactions are continuing within the facility," Chief Cabinet Secretary Hiromu Nonaka told an emergency news conference. "There are concerns about radiation in the surrounding areas."

He said that it was very likely that there had been a "criticality incident" at the plant.

Criticality is the point at which a nuclear chain reaction becomes self-sustaining.

"The situation is one our country has never experienced," Mr Nonaka said.

Three workers from the plant have been taken to hospital and hundreds have been forced to leave their homes.

One of the three workers in hospital is reported to be in a serious condition, suffering from continuous vomiting.

Prime Minister Keizo Obuchi has set up an emergency task force to tackle the accident.

A government request for help from US military forces in Japan for help was turned down. The US said its forces were not equipped to handle such accidents.

Blue smoke

The cause of the leak - detected at 1035 local time (0135GMT) - was not immediately known.

The head of the company's Tokyo office, Makoto Ujihara, said the workers told other staff at the plant that "they saw a blue flame rising from the fuel" and complained of nausea.

Plant manager Makoto Ujihara briefed the press about the accident

"We are still trying to find what exactly happened but we believe the uranium reached the critical point", the spokesman for JCO was quoted as saying.

Local schools were ordered to close their windows and keep pupils indoors.

The Prime Minister postponed a cabinet reshuffle planned for Friday because of the accident.

"Forbidden zone"

BBC Tokyo correspondent Juliet Hindell: "Nobody has accepted responsibility yet."

At a distance of two kilometers (1.24 miles) from the accident, radiation was still 10 times the normal level said Tatsuo Shimada, an official of Ibaraki Prefecture.

Police cordoned off a 6km "forbidden zone" around the uranium processing plant.

The International Atomic Energy Agency (IAEA) said that initial reports suggested a radiation leak in Japan was not a "major incident," although it was waiting for more data.

Early estimates suggested the incident was serious but would not rank above three on a seven-level scale of nuclear incidents, said an IAEA spokesman in Vienna.

The environmental organisation Greenpeace criticised the accident as a symptom of a safety "crisis" in Japan's nuclear industry.

"Today's accident at Tokaimura confirms our fears - the entire safety culture in Japan is in crisis and the use of dangerous plutonium in reactors here will only increase the probablity of a nuclear catastrophe," Greenpeace International activist Shaun Burnie said.

The organisation pointed out that the accident came just one day before a UK-flagged ship was expected to deliver 225 kilograms (495 pounds) of mixed plutonium-uranium oxide (MOX) fuel to a plant in Takahama, central Japan.

History of accidents

Tokaimura was the site of Japan's worst nuclear plant incident in 1997, when 35 workers were contaminated by radiation after a fire at a processing plant was not extinguished properly and caused an explosion.

A series of incidents at Japanese nuclear power stations in recent years has undermined confidence in the safety of this form of energy production, says BBC Tokyo correspondent Juliet Hindell.

In July, cooling water leaked from a pipe in the building that houses the reactor at the Tsuruga nuclear power plant in northern Japan.

It took Japan Atomic Power, the company that operates the plant, 14 hours to shut down operations after the leak was discovered.

Executives in charge of the reactor said radiation from the leak was 11,500 times the safety limit.

The earlier figure given was 250 times the limit, and the change has sparked accusations of a cover-up.

Nuclear programme

Japan has 51 commercial nuclear power reactors that provide one-third of the country's electricity.

With few natural resources of its own, Japan imports nearly all its fuel oil.

Since the oil crisis of 1973, successive governments have made concerted efforts to become self-sufficient.

By the year 2010, Japan wants to produce 42% of its energy in nuclear plants.

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