"If you are running a marathon, you have to keep going. You can maybe slow down a little bit," Pietro Barabaschi, director-general of the International Thermonuclear Experimental Reactor (ITER) project, told reporters.

Installed at a site in southern France, the project aims to prove the feasibility of fusion as a large-scale and carbon-free source of energy.

The decades-old plan has a history of technical challenges and cost overruns.

"I think it's important to keep a certain pace in research," Barabaschi said, comparing the current international context to "trying to cross rough sea".

ITER was set in motion after a 1985 summit between US president Ronald Reagan and Soviet leader Mikhail Gorbachev.

Its seven partners are China, the European Union, India, Japan, South Korea, Russia and the United States.

Russia still participates despite Western sanctions over its campaign in Ukraine.

When asked about the impact of tensions between Russia and the West, he said "of course we have some difficulties," citing as examples administrative issues and shipping components, without being more specific.

Barabaschi said there was "no such impact in the way we execute the project".

"It is a source of pride that we can show that engineers and scientists can still work together," he said.

Fusion entails forcing together the nuclei of light atomic elements in a super-heated plasma, held by powerful magnetic forces in a doughnut-shaped chamber called a tokamak.