“They will kill us because we are Nuer!” shouted a young man trying to get through  the gate of the United Nations base in Bor, South Sudan when I was there in early November. The base has been sheltering thousands of people, mostly ethnic Nuer. Due to recent construction, UN staff were closing the gate earlier than usual, and the young man feared being caught out of the safe haven in the majority Dinka town.

It has been almost a year since widespread ethnic fighting began in South Sudan, the world’s newest country, with almost 2 million people still displaced, including 100,000 who have taken shelter in UN bases. Across the conflict area, months of fighting and attacks on civilians, including ethnically motivated revenge killings, have only worsened ethnic tensions. A UN-supported arms embargo is urgently needed to stop the flow of the weapons being used against civilians by both sides. And the United States “holds the pen,” in UN-speak, playing a decisive role in determining UN action toward South Sudan.

Some 19,000 frightened people fled to the base in Bor last  December after fighting broke out in  Juba, the capital,  between Dinka members of  the presidential guard loyal to president Salva Kiir and those loyal to former vice president Riek Machar, a Nuer, who was fired earlier in 2013. Although triggered by a political dispute, much of the ensuing violence in Juba, Bor, and elsewhere took place along ethnic lines and targeted civilians, tens of thousands of whom fled their homes and sought refuge in UN bases.

The population at the base in Bor has dwindled to only 2,722, mostly Nuer, but their fear of the outside is still palpable. An attack on the Bor base in April by armed Dinka youth who fired indiscriminately on civilians killed about 50 people before UN peacekeepers stopped the assault. Local authorities, some of whom were present during the attack, have not investigated the slaughter, and the attackers presumably are still free and living in the community. The attack was apparently in revenge for earlier brutal killings of Dinka civilians by Nuer forces.

Just a short motorbike ride down the road from the UN base is Bor Town, a collection of low office buildings, a small market, and housing enclaves scattered around the town center. Most of the UN camp residents used to live there. The colleague who was with me in Bor had last been there in January to document the killings of Dinka civilians in December and January, many of them by the Nuer opposition forces.  As we followed the route she had taken earlier in the year, she pointed out where decomposing bodies had been strewn about the town and the site of mass graves.

South Sudan became independent from Sudan in July 2011, following a referendum and decades of civil war. The United States has provided significant support for South Sudan, including over $700 million in 2014 in humanitarian aid. The US has also provided $300 million in military assistance to South Sudan since 2005 – aid that was quietly suspended earlier this year after the fighting broke out. But the current crisis has left the US and other donors struggling to meet massive humanitarian needs.

The US also supports the UN peacekeeping mission in South Sudan, a vital player in protecting civilians, to the tune of $635 million since its inception in 2011. But the peacekeepers cannot solve all of South Sudan’s problems.

For months, the warring parties have engaged in peace talks and pledged cease-fires, facilitated and monitored by the Intergovernmental Authority on Development (IGAD), a regional body in East Africa. But as the dry season begins allowing fighting to resume, many thousands of civilians are still unable to go home.

The United States has imposed individual sanctions on senior military commanders from both sides, and recently announced that it would support UN sanctions at the Security Council. But individual sanctions, while important, will have limited impact. Government forces have received weapons from outside the country and reports indicate opposition forces have as well; only a UN-enforced arms embargo can put an end to large-scale weapons sales. While small arms will probably continue to make their way across borders, an arms embargo could stop a significant percentage of heavy weaponry that causes significant harm to civilians and their property.

Despite the attacks on civilians, the US and other members of the UN Security Council have hesitated to impose an arms embargo on South Sudan – ostensibly waiting for leadership from the region. They now have it: after the latest round of talks, IGAD announced on November 7 that a failure for the two parties to make peace before the end of the month would prompt travel bans, asset freezes, and arms embargoes. The regional authority called on the African Union, the UN, and the entire international community to support these measures.

Even if the parties do make progress at peace talks, an arms embargo is still necessary as ethnic tensions will remain high for some time, increasing the potential for significant violence against civilians. A UN-appointed panel of experts to monitor the embargo could provide a regular and detailed analysis of fresh arms supplies and violations of international human rights and humanitarian law, and hold any country that violates the ban accountable.

At the UN base, a UN police officer told us that the UN had agreed to keep the interior gate open through the weekend until the camp leaders and UN staff could meet to try to resolve the issue. No doubt the gate issue will be addressed. But for the people taking refuge at the UN base in Bor – and the tens of thousands of others around the country – the question of when, or even if, they can return home remains unresolved.

Prasow is deputy Washington director at Human Rights Watch.