The EU's executive arm warned Tuesday that Europe would go ahead and build the International Thermonuclear Experimental Reactor (ITER) in France without Japan if an agreement with Tokyo is not reached "as soon as possible".

Five nations and the European Union have been cooperating to build the reactor, a test bed for what is being billed as a clean, safe, inexhaustible energy source of the future.

"(The EU's) negotiating stance is worrisome and regrettable," said Takahiro Hayashi, deputy director of Japan's Office of Fusion Energy. But he added: "I think it would be good to agree on a site quickly."

The United States and South Korea back building ITER at Rokkasho-mura in Japan, while China and Russia support the European Union's site of Cadarache in southern France.

The ITER budget is projected to be 10 billion euros (13 billion dollars) over the next 30 years, including 4.7 billion euros to build the reactor. The European Union plans to finance 40 percent of the total.

The project, emulating the sun's nuclear fusion, is not expected to generate electricity before 2050.

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