Rebel gunmen in South Sudan massacred hundreds of civilians because of their ethnicity when they captured the key oil town of Bentiu last week, the UN said on Monday. It is one of the worst reported atrocities in the war-torn nation.

In the main mosque alone, “More than 200 civilians were reportedly killed and over 400 wounded,” the UN mission in the country said. “They (the rebels) searched a number of places where hundreds of South Sudanese and foreign civilians had taken refuge, and killed hundreds of the civilians after determining their ethnicity or nationality.”

Children were among the civilians slaughtered at a church, a hospital and an abandoned UN World Food Programme (WFP) compound, it said.

Fighters said on the radio that rival groups should be forced from the town of Bentiu and urged men to rape women from the opposition ethnic group.

Thousands of people have been killed in the northeast African nation since December, when presidential guards splintered along ethnic lines. The violence later spread across the country as soldiers loyal to President Salva Kiir tried to put down a rebellion led by former vice president Riek Machar.

Machar launched a renewed offensive this month targeting key oil fields. The conflict has taken on an ethnic dimension, pitting Kiir’s Dinka tribe against militia forces from Machar’s Nuer people.

The United States, the key backer of South Sudan’s move to independence , has threatened targeted sanctions against those responsible for the violence.