UNITED NATIONS — The United States said Thursday that it was ready to impose an arms embargo against South Sudan, a shift in position that many rights groups called long overdue to help stop the deadly violence convulsing the world’s youngest country.
The change coincided with warnings by the United Nations about the risk of genocide in South Sudan, where a civil war has been raging for three years.
The American proposal, near the end of the Obama administration, would halt the import of weapons to South Sudan, place travel restrictions on certain individuals deemed responsible for the violence and freeze their assets overseas.
The United States has not said how quickly it will propose the sanctions, nor whether they will include the country’s president, Salva Kiir; his rival and former vice president, Riek Machar; or their military commanders.
The American ambassador to the United Nations, Samantha Power, said she would introduce a draft resolution before the Security Council soon. “As we sit here, more arms are flowing into this country,” she told the Council.
Ms. Power’s remarks signaled that the Obama administration had lost patience with South Sudan’s rival leaders. She accused them of obstructing United Nations peacekeepers and of sending letters to United Nations headquarters with “happy talk” and “blatant misrepresentations.”
Human rights advocates have long called for targeted sanctions and an arms embargo against South Sudan. At first, the United States, which supported that country’s independence five years ago, was unwilling to take such action, saying privately that such punitive measures were tools of last resort.
Resistance to an arms embargo has also been expressed by other members of the Security Council, including Russia, which, like the United States, has veto power.
Russia’s deputy permanent representative, Petr Iliichev, said Thursday that an arms embargo would not help and described any sanctions as “the height of irresponsibility.” He said any new punitive measures would make it even harder for United Nations peacekeepers to operate.
China, which has peacekeepers in South Sudan, including those found to have failed to protect civilians during a grisly attack in Juba in July, said it supports “patient dialogue and consultation.” It was noncommittal on an arms embargo and suggested a “prudent” approach on sanctions.
The United States helped South Sudan gain independence from Sudan in 2011, after decades of war. Two years later, political rivalry between South Sudan’s two main leaders dragged the country into armed conflict. Each side whipped up ethnic animosity against the other’s communities.
The United Nations has for years documented harrowing crimes, along with the inability of its peacekeepers to stop them.
On Thursday, a grave warning was issued by the United Nations special adviser on the prevention of genocide, Adama Dieng, who said he had chronicled the “potential for genocide” in a visit to South Sudan last week.
In one anecdote, Mr. Dieng said an older man had told him he worried that his community “would be finished.” Mr. Dieng called for expanded targeted sanctions and an arms embargo.
Ms. Power referred to his warnings when she said: “None of us can say we did not see it coming. So the question for us is, ‘What will we do?’ ”
Secretary General Ban Ki-moon warned in a report this month that peacekeepers deployed in South Sudan would be unable to halt an escalation of violence. “There is a very real risk of mass atrocities,” the report said.
Rights activists welcomed the American shift. John Prendergast, director of the Enough Project, called it a “critical first step” to hold perpetrators accountable.