A top advisor to Russian President Vladimir Putin has quit his post at the Kremlin and fled Russia due to his nation’s invasion of Ukraine, two people familiar with the matter told Reuters.

Anatoly Chubais, a Kremlin special envoy who previously served as chief of staff to former President Boris Yeltsin, does not plan to return to Russia because of the conflict in Ukraine, one source said.

Chubais is the highest-ranking Russian official to publicly sever ties with Putin over the war with Ukraine.