The United States warned Tuesday that it will hold China responsible if Moscow makes gains in Ukraine, after Beijing renewed pledges of cooperation during a visit by Russia's top diplomat.

Kurt Campbell, the deputy secretary of state long associated with regearing U.S. policy toward Asia, said that for the United States, maintaining peace and stability in Europe is "our most important mission historically."

With Moscow renewing its Ukraine offensive amid a deadlock in the U.S. Congress on approving more weapons to Ukraine, Campbell warned that Russian territorial gains could "alter the balance of power in Europe in ways that are, frankly, unacceptable."