UNITED NATIONS — An American diplomat who helped shape the Obama administration’s efforts to steer South Sudan to independence in 2011 says he is appalled by the actions of its leaders, including those he personally knows.

“I’m horrified and disgusted by it,” said the diplomat, David Pressman, an alternate American representative to the United Nations. “How can you not be?”

Mr. Pressman leaves his post next week to become partner in the law firm of Boies, Schiller & Flexner.

“The leadership of this country were handed an incredible opportunity,” he said in an interview, along with what he called “huge amounts of assistance from the United States and from U.S. taxpayers.”

“The choices that have been made are the choices that have led to a fundamental shaking of the nascent foundations of the country, has resulted in innumerable killings, innumerable rapes,” he said.

Mr. Pressman spoke after the United Nations issued a highly critical report on Tuesday on the poor performance of its own peacekeeping mission in South Sudan in connection with a rampage of killing, looting and rape in July by government soldiers in the capital, Juba.

Some of the worst violence took place less than a mile from the peacekeeping force’s Juba headquarters, and the report said the peacekeepers did nothing to stop it.

Ambassador Samantha Power of the United States, who led a United Nations Security Council visit to South Sudan in September, also expressed outrage over the findings of the report.

“A number of civilians — including international aid workers — were gang-raped, beaten and threatened with death in front of others,” Ms. Power said in a statement on Thursday. “At no time during the attack did the U.N. send help to those in desperate need.”

She acknowledged the South Sudanese government’s announced plan to prosecute suspects implicated in the mayhem, calling it an “essential first step.”

The remarks by both American diplomats hinted at the difficulty that the United States has faced in helping resolve the political feud that broke out between South Sudan’s two main leaders, President Salva Kiir and his former vice president, Riek Machar, and quickly devolved into a full-on ethnic conflict between their supporters.

Since the conflict erupted in December 2013, tens of thousands of people have died and a United Nations report has warned of war crimes.

The Obama administration pushed to strengthen the United Nations peacekeeping mission, but even the mission’s 12,000 soldiers have been unable to protect civilians from some of the worst violence. Mr. Pressman said it had been “unable to function,” largely because of impediments imposed by the government, much like those established by the government of Sudan that the South Sudanese once fought. “They’re one-upping Khartoum in terms of what they’re doing,” he said, referring to Sudan’s capital.

After months, the Obama administration reluctantly backed an arms embargo, but ran into resistance from Russia, a veto-wielding member of the Security Council. In any event, Mr. Pressman said, an arms embargo would not solve the country’s crisis.

The report on the South Sudan peacekeeping mission also drew outrage from the government of Kenya, a major contributor, because Secretary General Ban Ki-moon ordered the dismissal of the force’s overall commander, a veteran Kenyan officer, who was in charge when the July mayhem took place.

Kenya announced on Wednesday that it was withdrawing its soldiers from the mission. It also accused Mr. Ban of scapegoating.

“There should have been a broader application of culpability,” Kenya’s United Nations ambassador, Macharia Kamau, said on Thursday. “There’s plenty of blame to go around.”

In a further act of retaliation on Friday, Kenya said it had deported a spokesman for the South Sudanese opposition, James Gatdet Dak, a registered refugee, back to South Sudan, provoking protests by United Nations officials who called it a violation of international law.

Mr. Gatdet Dak had expressed support for Mr. Ban’s dismissal of the Kenyan commanding officer, Lt. Gen. Johnson Mogoa Kimani Ondieki.