Explainer-How the U.S. could tighten sanctions on Russia over Ukraine

FILE PHOTO: Ukraine supporters protest against Russia's invasion of Ukraine during a demonstration near the White House in Washington, U.S., March 1, 2022. REUTERS/Kevin Lamarque/File Photo

WASHINGTON (Reuters) - The United States has imposed several rafts of sanctions on Russia over its invasion of Ukraine in February, targeting its central bank, major lenders, oligarchs and Russian President Vladimir Putin.

On Wednesday, Washington warned that the United States would take measures against Moscow in the coming days over "sham" referendums held by Russia in occupied regions of Ukraine.