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UPDATE 1-U.S. imposes sanctions on South Sudanese vice president

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WASHINGTON, Jan 8 (Reuters) - The United States on Wednesday imposed sanctions South Sudanese First Vice President Taban Deng Gai, the U.S. Treasury Department said in a statement, in Washington's latest move to pressure the country's politicians to form a unity government.

The Treasury Department said Gai reportedly arranged and directed the alleged killings of opposition politician Aggrey Idri Ezibon and human rights lawyer Dong Samuel Luak in a move to solidify his position in the government and intimidate members of the opposition.

Gai, the former governor of an oil-rich region in South Sudan, has long been a powerful government figure in the country with close ties to President Salva Kiir.

Gai is the latest South Sudanese official to be sanctioned by Washington. Last month, the United States blacklisted two South Sudanese cabinet ministers, days after slapping sanctions on five lower level officials it says are responsible for the likely murder of the two human rights activists in 2017.

The sanctions, implemented under the Global Magnitsky Human Rights Accountability Act that targets perpetrators of serious human rights abuses and corruption, freeze any of Gai's U.S. assets and generally prohibit Americans from doing business with him.

"Taban Deng Gai's attempt to silence the opposition party is derailing the countrys ability to implement a peace agreement," said Treasury Deputy Secretary Justin Muzinich in the statement.

Kiir and opposition leader Riek Machar signed a peace deal in 2018 to form the unity government by Nov. 12. But days before the deadline, the two leaders gave themselves an extension of 100 days to implement the agreement, a move criticized by Washington.

In November, the U.S. recalled its ambassador from South Sudan and said it was re-evaluating its relationship with the African country's government.

Civil war broke out in oil-producing South Sudan in 2013, less than two years after the country gained independence from Sudan following decades of war. The conflict has killed an estimated 400,000 people and forced millions from their homes. (Reporting by Daphne Psaledakis; Additional reporting by Maggie Fick in Nairobi Editing by Mary Milliken and Marguerita Choy)