An international project to build a nuclear fusion reactor in France that would start generating electricity in 17 years will face delays if U.S. cuts are not reversed in a few months, the head of the venture said on Tuesday.

Bernard Bigot, director-general of the International Thermonuclear Experimental Reactor project (ITER), said the Trump administration’s budget cuts would reduce the U.S. contribution to the project and lead to construction delays.

“It’s not a question of capability, it’s a question of political will,” Bigot said in a telephone interview while he was in Washington to meet with U.S. officials.

The Energy Department did not immediately respond to a request for comment.

ITER, launched 10 years ago by seven partners — the European Union, the United States, China, India, Japan, Russia and South Korea — would generate electricity from a process similar to the fusion that powers the sun.

Conventional fission reactors split unstable heavy atomic nuclei to generate power. They create poisonous nuclear waste, for which there is no permanent disposal site in the United States.

The cost of the experimental project, which its partners hope will reach its first full-power fusion by 2035, is $21 billion to $26 billion.

The United States has given about $1 billion to ITER so far and had been planning to contribute an additional $500 million through 2025.

But with President Donald Trump’s proposed budget cuts, Washington’s contribution for 2019 would drop to $75 million from $165 million. Last year, the administration cut the 2018 U.S. contribution to $63 million from a planned $120 million.

Bigot said he told officials from the Energy Department, the White House’s office of science and technology policy and the State Department that the project will face delays unless Washington promises by June to fund the original amounts.

If the United States, which provides 9 percent of ITER’s funding, does not contribute the planned tens of millions of dollars on time, contracts for assembling large parts of the project will be delayed, he said.

In December Shaylyn Hynes, an energy department spokeswoman, said that the administration is reviewing civilian nuclear policy, including research and development, which will influence its policy toward ITER going forward.