(Adds comments from U.S. officials, U.S. ambassador to U.N.; in paragraphs 4-7, 9, 14-16)
WASHINGTON, May 6 (Reuters) - The United States on Tuesday imposed sanctions on 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 announced sanctions against Peter Gadet, an army commander loyal to Machar, and Major-General Marial Chanuong, head of Kiir's presidential guard. The decision freezes any of their assets in the United States and blocks U.S. people or companies from dealing with them.
Chanuong led the operations in December in the South Sudanese city of Juba, where the latest bout of violence began. His troops led the slaughter of hundreds of civilians, U.S. officials said.
Gadet commanded troops responsible for violence that killed more than 200 civilians in the northern oil town of Bentiu in April, according to officials.
"We're using (sanctions) in a limited way against two individuals ... that we think are fairly significant, both of whom have blood on their hands," a U.S. official told reporters on condition of anonymity.
"We believe today's actions are significant, but also are a signal to any of those who would consider, or who are already contributing, to violence on either side in South Sudan."
Reuters 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.
"The measures taken against Marial Chanuong and Peter Gadet are only a first step and should serve as a clear warning to those in the government of South Sudan and those who have taken up arms against it: the United States is determined to hold accountable those who choose violence," U.S. Ambassador to the United Nations Samantha Power said in a statement.
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, and Power said the United States would support targeted U.N. sanctions.
It was not immediately clear what, if any, assets Chanuong and Gadet held in the United States. The Obama administration is also lobbying other countries to impose targeted sanctions, including Kenya and Uganda, where many South Sudanese own houses and property.
The U.S. official noted the sanctions would also affect attempted transfers through U.S. financial institutions.
"Typically a transaction between two African countries may well touch a U.S. institution and a transaction of that type would need to be blocked in the U.S.," the official said.
South Sudan became the world's newest state when it declared independence from Sudan in 2011, with strong U.S. support. (Reporting by Anna Yukhananov, additional reporting by Louis Charbonneau in New York and Phil Stewart in Washington; Editing by Chris Reese, Meredith Mazzilli and Mohammad Zargham)
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