JOHANNESBURG (AP) — The United States has imposed sanctions on South Sudan's First 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.

South Sudan's government could not immediately be reached for comment.

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.

More than 50 days into the 100-day extension, the U.S. statement said, “there have been no concrete steps to create the security conditions conducive to implementing a peace deal.” It added that the U.S. “will continue to apply pressure on the senior leadership of South Sudan to take concrete measures to bring peace and stability to the country."

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.

“Today's action represents a significant and important ratcheting up of U.S. financial pressure designed to break the impasse in South Sudan and hold those impeding peace accountable," John Prendergast, co-founder of The Sentry, said in a statement. The Washington-based investigative group has often reported on alleged corruption by South Sudanese officials.

“Over the past two years, the United States has sustained a campaign of pressure that is now appropriately directed at senior officials like Taban Deng, who have benefited financially and politically from conflict. These measures should continue until lasting peace is realized," Prendergast said.

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Matt Lee in Washington contributed.