"Our priority is to get an agreement with the largest number of participants and if possible with all six partners (the EU, the United States, Russia, China, Japan and South Korea).

"If there is no agreement, we'll have to think over ... how we go ahead with a maximum number of partners who want to participate," he told a news briefing.

Nuclear fusion has been touted as a long-term solution to the world's energy problems, as it would be low in pollution and use limitless sea water as fuel. The idea is to replicate the way the sun generates energy.

EU research and industry ministers are due to discuss how to move forward at a meeting on Nov. 25-26 and the Commission will recommend a course of action depending on the outcome of the Vienna talks, Fabbi said.

The EU's tactics in the fight for the $12 billion reactor resembled methods for which Europeans often criticize the United States - vowing to go it alone with a "coalition of the willing" if a multilateral forum does not back its course.

An EU source told Reuters this week that Cadarache was set to win the contest because Japan had signaled it would drop its bid in return for compensation.

But an official at the Japanese Science and Technology Ministry said Tokyo had not ended its bid to host the project.

Diplomats in Vienna said the outcome was still uncertain.

One Western diplomat familiar with the talks said: "They're still trying to slug it out. There may be a decision today on who gets it or they may decide that that they can't decide yet. The Japanese haven't given up yet."

"The Japanese are offering inducements to the French and the French are offering inducements to the Japanese," he said. The United States initially backed Japan's bid to put the reactor in the remote northern fishing village of Rokkasho in what was seen as a punishment to France for leading opposition to the U.S.-led invasion of Iraq.

It now appears to be neutral. Asked where Washington now stood, a U.S. official in Vienna said: "The United States supports a six-party ITER, but negotiations on where that will be located are still progressing today."

Fusion involves sticking atoms together, as opposed to today's nuclear reactors and weapons, which produce energy by blowing atoms apart.

However, 50 years of research have failed to produce a commercially viable fusion reactor.