The United Nations is condemning what it calls the "targeted killings" and wounding of hundreds of civilians based on their ethnic origins in the contested town of Bentiu.
The UN's mission in South Sudan said on Monday hundreds of civilians were reportedly killed or wounded after rebel forces took control of the capital of the oil-producing Unity state last week.
Toby Lanzer, the UN's top humanitarian official in South Sudan, said in Twitter posts late on Sunday that there were shocking scenes of atrocities, with "bodies of people executed'' lying in Bentiu's streets.
Lanzer told Al Jazeera on Monday that some individuals "associated with the opposition" had used an FM radio station to broadcast hate speech in the town.
|South Sudan's foreign minister speaks to Al Jazeera about the 'massacre' in Bentiu
“With hate speech and violence continuing as they are, we're going to have an even greater catastrophe on our hands at the end of this year," Lanzer said.
”“I think the saddest testament to the current situation is that we have had members of all communities, even those accused of perpetrating these crimes, fleeing to the UN base," he said.
“We had 5,000 civilians a week ago in our base, now we have 22,000 people. We have just one litre of portable water per person for today."
Thousands of people in South Sudan have been killed in violence and more than 1 million people have been forced to leave their homes since December when troops loyal to President Salva Kiir and those loyal to sacked vice president Riek Machar began to fight along ethnic lines.
“It is hard to believe that just a few months ago South Sudan was at peace," Lanzer said.
"People are on the brink of disaster. It is imperative the leaders recognise the crisis into which they have plunged their nation."