At least 16 people have been killed in South Sudan as militiamen targeted civilians based on their ethnicity.
Witnesses say the Dinka militiamen, aligned with South Sudan's government, went house to house Monday in the town of Wau searching for people from the local Luo and Fertit ethnic groups. Residents say streets were deserted Monday as families hid inside their homes.
The United Nations mission in South Sudan, or UNMISS, said its workers saw "the bodies of 16 civilians in a hospital. There were 10 people who had been injured."
U.N. officials have repeatedly warned that South Sudan is at risk of genocide.
U.N. officials say the killings were in retaliation for a rebel attack on government forces in Wau state on Sunday that killed two officers.
UNMISS said about 3,000 people, mostly women and children, had fled to a Catholic Church in town. It said 84 people had sought protection in a civilian encampment protected by U.N. peacekeepers. Residents told reporters that army soldiers were blocking the main road to the encampment, preventing most people from reaching the site.
South Sudan has been beset by violence for more than three years because of a political rivalry between the young country's two leaders.
The power struggle between President Salva Kiir, an ethnic Dinka, and his former deputy, Riek Machar, a Nuer, broke out in December 2013, after the president accused Machar and 10 others of attempting a coup.
Fighting has split the country along ethnic lines, displaced more than two million people from their homes, and caused food shortages in many areas.