Toby Lanzer told The Associated Press in a phone interview today that the ethnically targeted killings are "quite possibly a game-changer" for a conflict that has been raging since mid-December and that has exposed longstanding ethnic hostilities.
There was also a disturbing echo of Rwanda, which is marking the 20th anniversary this month of a genocide that killed 1 million people.
"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 Lanzer, who was in Bentiu on Sunday and Monday.
UN human rights investigators said yesterday that hundreds of civilians were killed last week because of their ethnicity after rebel forces seized Bentiu, the capital of oil-producing Unity state.
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Those rebel forces are Nuer, the same ethnic group that former Vice President Riek Machar, who is now a rebel leader, comes from.
Lanzer said thousands of civilians from several ethnic groups are streaming to the UN peacekeeping base in Bentiu because many believe more violence is coming.
The base now holds 25,000 people but has only one liter of water per person per day and only one latrine per 350 people.
As rebel forces entered Bentiu last week, residents were led to believe that by entering the city mosque they would be safe, Lanzer said, citing accounts from survivors.
But once inside they were robbed of their valuables, mostly money and mobile phones, and a short while later gunmen arrived and began killing, both inside the mosque and inside the city hospital.
British Ambassador Ian Hughes today said the April 15-16 killings are a clear violation of international law. He said those behind the atrocities and those inciting the killings will be held to account.