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WASHINGTON (Reuters) - The United States on Tuesday sanctioned two people on opposing sides of the ethnic violence in South Sudan, showing the growing frustration in Washington with leaders in an African country it helped create.

Thousands of people have been killed and more than 1 million have fled their homes since fighting erupted in the world’s newest nation in December between troops backing President Salva Kiir and soldiers loyal to his sacked deputy, Riek Machar.

The U.S. Treasury Department sanctioned Peter Gadet, an army commander loyal to the rebel leader Machar, and Major-General Marial Chanuong, head of Kiir’s presidential guard. The designation freezes any of their assets in the United States and blocks U.S. people or companies from dealing with them.

Reuters first reported on Monday that the United States expects to impose sanctions on individuals from both sides of the conflict, in a warning sign to curb violence top U.N. officials said could spiral into genocide.

South Sudan’s U.N. mission did not have an immediate response when contacted for comment on the U.S. move.

The sanctions are the first imposed under an executive order U.S. President Barack Obama signed in April allowing sanctions against those responsible for the violence in South Sudan.

Secretary of State John Kerry this week also threatened sanctions on South Sudan rebel leader Machar if he spurned peace negotiations. Machar on Tuesday said he would “try his best” to make it for peace talks.

Members of the U.N. Security Council are also considering targeted U.N. measures in addition to any unilateral steps Washington takes on South Sudan.