The United Nations has accused rebels in South Sudan of killing hundreds of people in a small town based on their nationality or ethnic origin.

The U.N. Mission in South Sudan says the slayings took place in Bentiu after rebel forces seized the town from government troops on April 15.

The mission says the rebels killed more than 200 and wounded more than 400 civilians at a local mosque where hundreds of people had taken shelter to escape fighting. It says the rebels escorted people of certain nationalities and ethnic groups to safety before killing the others.

The U.N. mission says the rebels, known as the SPLA in Opposition, also killed several people at a Catholic church and a vacated World Food Program compound after asking them to identify their ethnicity.

There was no immediate comment on the report from the rebels or South Sudan's government.

A U.N. official in South Sudan, Toby Lanzer, posted video of several bodies lying along a dirt road in Bentiu. He said on Twitter that thousands of people have fled the town in recent days and that 22,000 civilians are now being protected at a local U.N. compound.

The U.N. mission condemned the killings and mission chief Raisedon Zenenga said the perpetrators must be held accountable.

South Sudan has been wracked by fighting since mid-December, when the government accused accused a former vice president of leading a coup attempt.

Ethnic violence and clashes between rebel and government forces have killed thousands of people and displaced an estimated 1 million from their homes. Peace talks in Ethiopia have made little progress.