GAMBELA, Ethiopia — After angry mobs began targeting his community, Simon Thion, 29, felt caught in the middle.

Mr. Simon, an Ethiopian who is part of the Nuer ethnic group, went to a hospital in the western town of Gambela recently to visit his nephew, who was injured when members of the Murle ethnic group crossed into Ethiopia from South Sudan to steal cows and kill hundreds of Nuer villagers.

Now, he is afraid to leave the hospital. In the regional capital where he lives, he says, Nuer are targeted by other Ethiopians. “If I leave this compound, highlanders will come and kill me,” he said.

Highlanders is a term used to describe Ethiopians who trace their heritage to the country’s central regions, including the capital, Addis Ababa.

Tensions between Nuer and highlander Ethiopians in the Gambela region have been relatively low, but that changed this month when a highlander, a contracted driver for a nonprofit group, was accused of a hit-and-run that killed two children in a predominantly Nuer refugee camp near the town of Gambela.

Fury over the deaths incited a surge of retaliatory violence, in which Nuer killed about 10 highlanders around the camp.

Several highlanders who saw the bodies at a hospital said the killings were gruesome, and in recent days hundreds of them have joined with sympathetic members of the Anuak ethnic group, who say they are indigenous to Gambela, to demonstrate against Nuer, whom they consider intruders.

In South Sudan, a civil war that began in December 2013 has killed tens of thousands of people and displaced more than two million. More than 220,000 South Sudanese refugees and other asylum seekers, the vast majority of them Nuer, have spilled into crowded camps in the Gambela region of Ethiopia since then, tilting the demographic balance in an area where antagonisms between the Nuer and Anuak groups have long simmered.

At least one Nuer has been killed in the last few days and one wounded, according to Gatluak Tut Khot, the regional president of Gambela. Nuer in the area said the death toll was as high as 15.

The clashes occurred days after many Ethiopians united around the cause of rescuing an estimated 102 Nuer children kidnapped on April 15 by Murle raiders, who killed an estimated 208 Nuer Ethiopians. Last week, the government declared two days of mourning for those who were killed.

Melkamu Assefa, 20, a highlander in the town of Gambela, said he no longer sympathized with those victims. He expressed anger about the deaths of the highlanders near the refugee camp, and condoned the killings of Nuer, partly because he said the Nuer would like the Gambela region to be a part of South Sudan.

Tensions are rising at a pivotal moment. For a week, Riek Machar, South Sudan’s opposition leader and a former vice president, who is Nuer, has been engaged in tense negotiations with officials in the country’s capital, Juba. Mr. Machar’s aim is to fly there from Gambela Airport to be reinstated as a deputy to his rival, President Salva Kiir, as part of a peace deal.

But the highlanders demonstrating against Nuer in Gambela want little to do with South Sudanese politics. The recent clashes are about Ethiopian nationalism, Mr. Melkamu said. “In the Nuer part of town, they once burned an Ethiopian flag and raised the South Sudanese flag instead,” he said, adding that he supported any efforts to drive all Nuer — refugees or not — out of town.

Standing inside the hospital gates, Mr. Simon said, “They aren’t differentiating between Nuer who are Ethiopian citizens and those who are not.”

He added, “Of course I am afraid.”