The multibillion-dollar International Thermonuclear Experimental Reactor (ITER) project will either be sited in the French town of Cadarache or the northern Japanese village of Rokkasho-mura.
The project aims to be a test bed for what is being billed as the clean, safe, inexhaustible energy source of the future, emulating the sun's nuclear fusion. The project, however, is not expected to generate electricity before
"I am proud to say today that the United States strongly supports building ITER here in Japan. From a technical standpoint you offer the superior site," US Secretary of Energy Spencer Abraham told a gathering of Japanese business leaders during a visit to Tokyo.
"The location of Rokkasho is superbly situated to receive the large materials needed for ITER.
"Your technical and engineering skills are known and admired in every corner of the world. What is more, the local communinity clearly welcomes this project and has always gone out of its way to encourage the siting of ITER in Rokkasho," Abraham said.
A spokesman for the US embassy in Tokyo confirmed it was the first time that Washington had gone on record in support of the Japanese bid.
"We feel extremely encouraged by his comment," an official from Japan's science and technology ministry told AFP.
"We will continue to work with other countries so that we can build ITER in Rokkasho-mura."
The choice between Cadarache and Rokkasho-mura was supposed to have been made in December by delegates from the European Union, the US, China, Japan, South Korea and Russia at a meeting in Reston, Virginia, west of Washington.
But after they failed to reach a consensus, the decision was delayed, probably until mid-February.
The European Union, Russia and China supported the Cadarache site, while the US, Japan and South Korea preferred the Japanese site, according to a delegate present at the meeting.
A French source close to ITER told AFP it would have been better for Abraham to have waited for the completion of ongoing technical studies before commenting. "The remarks of the US secretary amount to a questionable way of disrupting the study," she said.
The source dismissed Abraham's endorsement of Rokkasho's coastal location for the easier delivery of large components, arguing its exposure to the risk of earthquake and tidal waves was more important.
The Japanese site has many assets including the close proximity of a port, a ground of solid bedrock and a nearby US military base.
Rokkasho-mura is already home to a nuclear reprocessing facility due to start operating in July 2006, a uranium enrichment facility and low and high-level nuclear waste facilities.
The French site offers an existing research facility and a better climate.
Lobbying for the world's first nuclear fusion reactor has been intense as it will provide an economic boom to its chosen site. French officials have estimated the project could bring 30 billion dollars to the economy of the chosen venue over 30 years.
It will cost five billion dollars to build the reactor, and five billion to run it for 10 to 20 years, according to project supporters.
The choice of the site must be made by consensus, and not by a simple majority, partly because all parties will be required to fund the project.