The United States on Wednesday imposed sanctions on 64 companies and three Bulgarians over corruption, including an oligarch accused of planning to create a conduit for Russian political leaders to influence the Bulgarian government.

A statement by the Treasury Department said the move was its single biggest action targeting graft to date.

Bulgaria ranks as the European Union's most corrupt member state, according to Transparency International. The Balkan country has repeatedly been criticized by the European Commission for failing to root out corruption and send a single high-ranking senior government official behind bars for graft.

Bulgarian interim Prime Minister Stefan Yanev said he was informed by U.S. Under Secretary of State for Political Affairs Victoria Nuland by telephone about the move, part of efforts to effectively combat corruption in Bulgaria.