Russian President Vladimir Putin will face “substantial consequences” if he deploys forces into Belarus to help beleaguered autocrat Alexander Lukashenko control the mass protests against his rule, a top American diplomat warned.

“There would be substantial consequences for the relationship between Russia and the United States, between Russia and Europe,” Deputy Secretary of State Stephen Biegun told Radio Free Europe/Radio Liberty, a U.S.-backed outlet. “The last four years has been very challenging for U.S.-Russian relations, but it is possible that it could be worse.”