The United States has "credible information" that South Sudan's government is targeting civilians in Central Equatoria state and preparing for "large-scale" attacks within days and weeks, a top U.S. diplomat said Wednesday.

U.S. ambassador to the Human Rights Council Keith Harper told a special session of the U.N. body in Geneva that the South Sudanese government has mobilized at least 4,000 militia members and "is staging these fighters in Equatoria to begin conducting attacks" — a claim quickly denied by his counterpart from the African country.

"The United States is alarmed by recent reports about the situation in South Sudan," Harper said, noting that over 1,900 homes in Central Equatoria had been destroyed over the last two months. "We have credible information that the South Sudanese government is currently targeting civilians in Central Equatoria and ... preparing for large-scale attacks in the coming days and weeks."

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Speaking to The Associated Press after the session of the 47-member council, Harper said that U.S. diplomats were "principally working it through New York because that is where we can do an arms ban, arms embargo, individualized multi-lateral sanctions. So all of these things are on the table."

On her Twitter account, U.S. Ambassador to the United Nations Samantha Power late Tuesday called the troop increase a "huge red flag," saying that the Security Council "must halt" the flow of arms to South Sudan.

During Wednesday's session in Geneva, South Sudan ambassador Kuol Alor Kuol Arop didn't directly reject Harper's claims about a new, upcoming offensive, but responded that the country's vice president and other top officials are part of the transitional government. Speaking to the AP, Arop denied any build-up of forces or plans for an offensive.

Adama Dieng, the U.N. secretary-general's adviser on the prevention of genocide, has said that Central Equatoria and the Yei region are at risk of genocide. A recent AP investigation uncovered evidence of ethnic targeting and frequent attacks on civilians by South Sudan's military.

According to the U.N. High Commissioner for Refugees, more than 300,000 South Sudanese have fled into Uganda since fighting erupted in the capital, Juba, in July.

On Monday, South Sudan's government blocked a team of international monitors from traveling to Central Equatoria to investigate violations of the country's peace deal.