The head of Holtec, a private American nuclear power company that operates in Ukraine, said that Russia’s seizure of the Zaporizhzhia nuclear power plant was a serious blow to the future of clean energy, Reuters reports.

Russia seized Europe’s largest plant shortly after invading Ukraine on February 24, 2022.

During the war, the plant suffered from bombings and downed power lines, raising concerns about a nuclear disaster. Russia and Ukraine blame each other for the explosions.

Chris Singh, CEO of New Jersey-based Holtec International, said in an open letter published Thursday that Russian President Vladimir Putin’s military occupation of the plant should be viewed as “a major blow to humanity’s clean energy future.”

Singh said it had “normalized a new, monstrous instrument of war”.

Proponents of nuclear power, including the Biden administration, say the power source is critical to combating climate change because it produces electricity with virtually no emissions.

Critics of nuclear power say building new plants is too expensive and time-consuming to significantly reduce climate change, although some agree that current reactors should continue to operate if they operate safely.

In Ukraine, Holtec is working on a repository for spent nuclear fuel and wants to build new generation small modular reactors there.