The United States urged the U.N. Security Council on Thursday to impose an arms embargo and new sanctions on South Sudan to curb violence that could lead to mass atrocities, a proposal immediately rejected by Russia as premature and irresponsible.

U.S. Ambassador Samantha Power told the council that an arms embargo can't prevent weapons getting into the conflict-wracked country, but it would be "an important step toward curbing the ongoing violence perpetrated by government and opposition forces against civilians."

Russia's deputy U.N. ambassador Petr Iliichev countered that an arms embargo "would hardly be helpful in settling the conflict," and imposing a travel ban and asset freeze on South Sudan's leaders "would be the height of irresponsibility now."

Their exchange followed a plea by the U.N. expert on preventing genocide, Adama Dieng, for action including an immediate arms embargo to help keep ethnic hatreds in the world's newest nations from evolving into genocide.

He told the council that conversations with those involved in the conflict during a visit last week "confirmed that what began as a political conflict has transformed into what could become an outright ethnic war."

The draft resolution, obtained by The Associated Press, would prevent "the direct or indirect supply, sale or transfer" of weapons to the government or the opposition, and would add individuals to the sanctions list. Power said the people targeted would be those most responsible for preventing peace.

There were high hopes that South Sudan would have peace and stability after its independence from neighboring Sudan in 2011. But the country plunged into ethnic violence in 2013 when forces loyal to President Salva Kiir, a Dinka, started battling those loyal to his former vice president Riek Machar, a Nuer.

A peace deal signed in August 2015 has not stopped the fighting. Tens of thousands of people have been killed, more than 2 million are displaced, and the U.N. envoy for South Sudan, Ellen Margrethe Loj, told the council the country faces a "dire humanitarian situation" with 4.8 million people estimated to be "severely food insecure."

Loj said the deteriorating economy "and the increasingly fragmented conflict, often with ethnic undertones, ... have placed the country on a potential downward slide towards greater divisiveness and risk of a full-scale civil conflict that could render national cohesion almost impossible to achieve."

But she said it is important not to look at the conflict as only between supporters of Kiir and Machar, saying there are 64 different ethnic groups in the country, many with differences, as well as criminal elements involved in the upsurge in violence since a wave of attacks in the capital, Juba, in July.

The Security Council adopted a resolution in August authorizing 4,000 troops from African nations to beef up the 14,000-strong U.N. peacekeeping force in South Sudan and said if the government didn't comply it would consider an arms embargo.

Secretary-General Ban Ki-moon said in a report circulated Wednesday that it has taken the U.N. "almost three months to secure what is essentially piecemeal and limited consent" for the additional African troops.