The United Nations says hundreds of civilians in South Sudan were massacred last week in the city of Bentiu.
Many lost their lives after taking shelter in a mosque where they were trapped after rebel forces seized control of the town from government troops.
A U.N. official described "piles and piles" of bodies inside the mosque, in a hospital, and on the streets.
Thousands of people have been killed in South Sudan since December, when presidential guards splintered and fought along ethnic lines.
The violence later spread across the country as soldiers loyal to President Salva Kiir, an ethnic Dinka, tried to put down a rebellion led by former Vice President Riek Machar, an ethnic Nuer.
In a phone interview Tuesday, the official told The Associated Press the mass killings carried out by Nuers on April 15 and 16 are "quite possibly a game-changer" in the conflict.
"It's the first time we're aware of that a local radio station was broadcasting hate messages encouraging people to engage in atrocities," said Toby Lanzer, who was in Bentiu on Sunday and Monday.
"And that really accelerates South Sudan's descent into an even more difficult situation from which it needs to extract itself," Lanzer said.
Lanzer said thousands of civilians from several ethnic groups are streaming to the U.N. peacekeeping base in Bentiu because many believe more violence is coming.
Nearly 22,000 people have taken shelter at the base, up from 4,500 at the start of April. Each person must make due with one liter of water per day, with some 350 people sharing one toilet.