State Department spokesman Sean McCormack said Belarus and Cuba were "two of the world's most active perpetrators of serious human rights violations."
McCormack criticized the council's new rules that permitted special scrutiny of Belarus and Cuba to be halted.
The United States, which has previously expressed concerns about the council, is only an observer, not a member, of the body.
Earlier, the current special rapporteur for Belarus, whose post will be abolished under the new rules, said the council's move sent a "wrong and worrying" signal to Minsk.
Adrian Severin, a Romanian law expert, said President Alyaksandr Lukashenka's authoritarian government would react with triumph to this and victims of repressions would lose "a necessary tool" to defend their rights.
The move came at a meeting of the 47-nation council in Geneva on June 18.
Russia is reported to have led demands for the end of the mandate of the monitor for Belarus.
Belarus and Cuba are accused of human rights abuses, particularly of political rights.
UN Human Rights Council
UN General Assembly delegates applaud the creation of the UN Human Rights Council on March 15, 2006 (epa)
A FRESH START ON HUMAN RIGHTS: The United Nations General Assembly on May 9 elected members to its new Human Rights Council, a step that reformers hope will help improve the United Nations' sullied record on defending human rights. The UN's old human rights watchdog -- the Commission on Human Rights -- had long been criticized for granting membership to countries with dismal human rights records, such as Cuba, Sudan and Zimbabwe.
Every member of the new body has to pledge to promote human rights. (more)