UN chief Ban Ki-moon has urged the Security Council to add 5,500 UN troops to the 7,000-strong force in South Sudan, amid escalating fighting there.
His plea comes as new details emerge of alleged ethnic killings committed during more than a week of violence.
Mr Ban warned that anyone responsible for abuse would be held to account.
Tens of thousands of people have fled fighting, as rebels thought to support sacked former vice-president Riek Machar have seized major towns.
A journalist in the capital Juba, Hannah McNeish, said witnesses had told her about a massacre in which more than 200 people, mostly from the Nuer ethnic group, were herded into a police station and shot by security forces.
Another man interviewed at the UN base in Juba reported that gunmen from the Dinka tribe were shooting people in Nuer districts who did not speak the Dinka language.
The allegations cannot be independently verified. Up to 1,000 people are thought to have been killed in the fighting and UN compounds are sheltering more than 40,000 civilians.
UN humanitarian co-ordinator Toby Lanzer, who was in Bor, north of Juba, over the weekend, told the BBC he had witnessed “some of the most horrible things that one can imagine”.
He said “people were being lined up and executed in a summary fashion”.
‘Face the consequences’
The conflict began last week, when President Salva Kiir, a member of the majority Dinka ethnic group, accused Mr Machar, a Nuer, of attempting to launch a coup.
Mr Machar, who was sacked as vice-president in July, denies trying to seize power.
The fear is that their personal rivalry will spark a full-scale conflict between the Nuer and Dinka groups.
NB: Sudan’s arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.