US Ambassador Samantha Power said President Salva Kiir's government must quickly show that it will follow up on its commitment to allow the 4,000-strong force to be deployed in Juba, the country's capital.
"If the government of South Sudan does not allow the regional protection force to deploy or does not allow the UN to move in a way that it needs to move to protect civilians, the United States certainly will support an arms embargo," Power told reporters.
The Security Council was meeting behind closed doors to discuss plans for the new force and ongoing reports of South Sudanese soldiers stopping UN peacekeepers at checkpoints and forcing them to return to their bases.
The council voted last month to deploy the regional protection force (RPF) in Juba, which will be under the command of the UN peacekeeping mission.
African leaders called for the force after heavy fighting engulfed Juba in early July, setting back efforts to end the devastating war that has raged in South Sudan since December 2013.
After initially opposing the force, Kiir this month agreed to the deployment during talks with council ambassadors who traveled to Juba for meetings with the leadership.
"We recognize that the commitments that were made can't bear fruit necessarily overnight, but they also can't take days and days to bear fruit," said Power.
"We need to see concrete progress in the deployment of the RPF and concrete progress on securing movement for UN peacekeepers so that civilians who are in desperate need of protection can find it with the UN," she said.
South Sudan descended into war in December 2013 after Kiir accused his former deputy Riek Machar of plotting a coup
UN Secretary-General Ban Ki-moon told the council in a report last week that the first advance teams of the new force could begin arriving in Juba by the end of this month.
Ban said he would report to the council in October on whether South Sudan's government is cooperating with the United Nations on the deployment of the regional force.
If he finds it is not cooperating, that would trigger a vote at the council on the proposed arms embargo.
South Sudan descended into war in December 2013 after Kiir accused his former deputy Riek Machar of plotting a coup.
During the fighting in July, Machar, who had been persuaded to return to Juba to join a national unity government agreed under a peace deal, fled the country and is now in Khartoum, having been replaced by Taban Deng Gai in Juba.
Aside from the tens of thousands of people killed in the conflict, the United Nations has reported shocking levels of brutality including gang rapes and the wholesale burning of villages.