The United States is urging the U.N. Security Council 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 Thursday that an arms embargo would be "an important step toward curbing the ongoing violence perpetrated by government and opposition forces against civilians."

But Russia's deputy U.N. ambassador Petr Iliichev said an arms embargo "would hardly be helpful in settling the conflict" and sanctions against South Sudan's leaders "would be the height of irresponsibility now."

Their exchanged followed a U.N. expert's plea for the council to immediately impose an arms embargo to help keep ethnic hatreds in the country from evolving into genocide.