JOHANNESBURG (AP) — The United States has imposed sanctions on South Sudan's Vice President Taban Deng Gai, citing his involvement in serious human rights abuses.

The statement Wednesday is one of the sharpest actions taken by the U.S. against the South Sudan government amid Washington's frustration over the country's failure to achieve a stable peace after its civil war.

The Treasury Department statement asserts that Deng has been involved in the disappearance and deaths of civilians. It also asserts that he has acted to divide the armed opposition in South Sudan, extending the country's five-year civil war that killed nearly 400,000 people.

The U.S. is among the countries pressuring South Sudan's rival leaders to form a coalition government. A deadline for that looms next month after the parties failed to make one in November,

The U.S., which backed the fight for South Sudan's independence from Sudan, achieved in 2011, has been increasingly frustrated by the inability of President Salva Kiir and rival Riek Machar to reach a lasting peace. Meanwhile, the government faces widespread accusations by critics of mismanagement and corruption.