A decision on where to site the world's first big nuclear fusion reactor has been postponed until next year.
ITER - NUCLEAR FUSION PROJECT
Project estimated to cost 10bn euros and will run for 35 years
It will produce the first sustained fusion reactions
Final stage before full prototype of commercial reactor is built
Officials from several countries meeting in Washington were divided on whether to build the international reactor in France or Japan.
The US has been against the French option because of France's opposition to the US-led invasion of Iraq.
Nuclear fusion holds out the promise of virtually limitless - and largely pollution-free - energy.
Experts say the country hosting the International Thermonuclear Experimental Reactor (Iter) project will gain a potentially lucrative head start in expertise and technology.
Pros and cons
Representatives from the US, EU, Japan, Russia, China and South Korea were hoping to make a final decision on the location of the project.
"At the end of the meeting... it was agreed by all parties present that no definite choice could be made at this stage," France's research ministry said in a statement.
A French Government envoy at the meeting, Pierre Lellouche, said the matter would be deferred until next year, probably mid-February.
The Japanese site of Rokkasho-mura has the advantages of proximity to a port, a ground of solid bedrock and a nearby US military base.
The French site at Cadarache offers an existing research facility and a more moderate climate.
The experts were supposed to reach a consensus based on objective criteria.
THE ITER CONSORTIUM
But BBC News Online science editor Dr David Whitehouse says the decision is highly political, involving huge amounts of horse-trading behind the scenes.
Delegates were hoping, he says, that one of the countries involved would drop out and so avoid the need for a vote.
The European Union, Russia and China are known to be backing France - but South Korea, the United States and Tokyo itself are reported to be favouring Japan.
The US, in particular, has raised objections to the French option, citing its opposition to the Iraq invasion.
"We have the structure, scientific and technical environment to ensure that this scheme can start up with competence, expertise and solid safety guarantees," French Research Minister Claudie Haignere said.
"If our site is chosen, Japan will cover the costs that are needed," said Hidekazu Tanaka, a senior official of the Japanese Education, Culture, Sports, Science and Technology ministry.
Iter is the boldest nuclear initiative since the Manhattan Project - the effort to build the first atom bomb - says Dr Whitehouse.
It would also be the world's largest international co-operative research and development project after the International Space Station.
Scientists say it will be the first fusion device to produce thermal energy at the level of a conventional electricity-producing power station.
Its goal will be to produce 500 megawatts of fusion power for 500 seconds or longer during each individual fusion experiment and, in doing so, demonstrate essential technologies for a commercial reactor.
But all are agreed that taming the power of the Sun will not be easy.
The superhot gas in which the fusion takes place is notoriously difficult to control.
The gas, termed a plasma, has to be contained in a magnetic field or it will damage the reactor. So far, no one has achieved a prolonged self-sustaining fusion event.
Advocates of fusion power point out that if they succeed, there is an almost limitless supply of power available because the deuterium atoms on which it would be based can be derived from seawater.
In nuclear fusion, atoms are forced together, as opposed to nuclear fission in which energy is released by splitting atoms.