UN's Ban Ki-moon seeks to boost South Sudan force
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.
In letter sent to the Security Council on Tuesday, Mr Ban called on members to strengthen the UN mission in the country, Unmiss, "on an urgent basis in order to help ensure the protection of civilians and the protection of United Nations personnel".
He requested that 5,500 troops be reassigned from UN missions in other African countries, including Liberia and Democratic Republic of Congo.
In addition, he asked for hundreds more police, three attack helicopters, three transport helicopters and one military transport plane.
Earlier he said would "investigate reports of grave human rights violations and crimes against humanity".
"Those responsible at the senior level will be held personally accountable and face the consequences, even if they claim they had no knowledge of the attacks," Mr Ban added.
Two Indian peacekeepers were killed last week in a rebel raid on a UN compound.
The fighting began in the capital Juba last week after Mr Kiir said he had quashed an attempted coup.
Since then, violence has spread throughout South Sudan, with rebels taking the major towns of Bor and Bentiu.
President Kiir told parliament earlier that he was willing to hold talks with Mr Machar, saying that a delegation of East African foreign ministers had offered to mediate.
However, he said that Mr Machar would have to come to the table without any conditions.
Mr Machar told Reuters news agency that he was open to dialogue if his political allies were released from detention.
Over the weekend, the US deployed extra troops to help evacuate Americans and other foreigners.
In Bor, three US military aircraft were fired upon on Saturday, forcing the evacuation to be aborted. On Sunday, the US re-entered using civilian US and UN helicopters.
Sudan suffered a 22-year civil war that left more than one million people dead before the South became independent in 2011.