South Sudan, the world's newest country, is in its fifth year of war following a 2013 split between President Salva Kiir and his former deputy Riek Machar. It's mostly symbolic since the US has nearly no defense trade with the East African country in the first place.
In South Sudan's capital Juba, Kiir's spokesperson Ateny Wek Ateny declined to comment.
Amnesty International has documented evidence of violations and abuses that have been committed by government and opposition forces with complete impunity, brutality and a disregard for human life.
The United States has announced that it was implementing restrictions on the export of defence articles and defence services into South Sudan. Officials say more than a million South Sudanese are on the brink of starvation, 2.4 million refugees are fleeing to neighboring countries and almost 2 million have been internally displaced.
"We encourage IGAD and the African Union to consider sanctions measures against those who undermine the peace process", Nauert said in the statement. "We will continue to take actions against those who foment violence and obstruct the peace process", Nauert said.
The United States is seeking a UN Security Council arms embargo on all weapons flowing into South Sudan.
Any US push for the U.N. Security Council to take further action against South Sudan is likely to be resisted by veto power Russian Federation.
The United States is South Sudan's biggest aid provider, and was a major backer of its 2011 independence from Sudan. U.N. Ambassador Nikki Haley told the U.N. Security Council last week an arms embargo could "slow the violence, slow the flow of arms and ammunition", to South Sudan.
The US announcement comes four days after the African Union said it was open to imposing sanctions on leaders violating the ceasefire agreement in South Sudan.
Late last month, USA ambassador Nikki Haley announced that the United States had given up on South Sudan's leader after investing more than $11 billion in the country, and she called President Salva Kiir an "unfit partner" in the pursuit of peace.
Following his visit to Ethiopia and Somalia, UN humanitarian chief Mark Lowcock will join the High Commissioner for Refugees, Filippo Grandi, on a visit to Kakuma refugee camp and jointly launch the South Sudan Humanitarian Response Plan and the Regional Refugee Response Plan in Nairobi on 1 February.