The American ambassador to the United Nations, Samantha Power, made her name by arguing for intervention in countries to avert genocide and recently returned from a visit to another strife-torn country, the Central African Republic. At the United Nations, she drafted a resolution to increase the number of peacekeepers in South Sudan.

Secretary of State John Kerry witnessed the referendum while a senator — and has reminded South Sudan’s president, Salva Kiir, of that in repeated phone calls urging him to reconcile with his former vice president, Riek Machar. Their political feud revived barely dormant ethnic tensions within the country.

The United States, a senior administration official said, is getting “a lot of very disturbing reports about targeted killings of Nuer, as well as targeted killings of Dinka” — the two main ethnic groups in South Sudan. The specter of mass atrocities has rattled the administration, which on Friday pledged an additional $49.8 million in humanitarian aid for the roughly 180,000 people driven from their homes by the fighting.

“We can’t allow the carnage to go on; we can’t allow the capital to be overrun,” said Tom McDonald, who worked on Sudan issues as the American ambassador to Zimbabwe during the administration of Bill Clinton. “We have too much to lose; we’ve put too much into this.”

Mr. McDonald, now a lawyer at Baker Hostetler, said the administration should pursue a dual track, encouraging reconciliation while developing plans to help Uganda and Ethiopia with military intervention in the event that diplomacy fails. Uganda’s president, Yoweri Museveni, has threatened to intervene if the rebels do not agree to a cease-fire.

While the United States is highly unlikely to commit its own troops, Mr. McDonald said, it could provide planes to transport Ugandan soldiers or share intelligence on rebel positions with Ethiopia’s air force. American officials said that they hoped that Uganda’s warning would be enough of a deterrent and that intervention would not be needed.

In the meantime, said Grant T. Harris, the senior director for African affairs at the National Security Council, the United States is pushing to expand the size and mission of the United Nations’ peacekeeping force, which currently numbers more than 7,600 and is struggling to protect an estimated 45,000 refugees who have swarmed its compounds.