The United Nations says the South Sudan conflict death toll is "very substantially" higher than the figure of 1000 given so far and is stepping up efforts to shelter civilians.
The UN mission in South Sudan has cut all military links with the government army and is ready to act against any attack by the army on its bases, UN officials said.
Most concern is now focused on the oil town of Bentiu, where rebel forces loyal to former vice president Riek Machar are holding off the army of President Salva Kiir.
UN deputy spokesman Farhan Haq said on Thursday that the outcome of the battle for Bentiu, capital of Unity state, was "unclear and fluid".
"Explosions and fighting have been reportedly heard in the town this morning," said Haq, who added that more than 8000 people have fled to the Bentiu UN compound.
The mission was expanding its protection site at Bentiu to cope with the growing numbers seeking shelter there, he said.
There are now more than 60,000 people at UN compounds across South Sudan, half of them in Juba and another 9000 in Bor, the rebel held capital of Jonglei state.
And the number of South Sudanese taking refuge in Uganda has more than tripled to 32,000 in the past two days, the UN said, adding that more than 10,000 are also registered in Kenya and Ethiopia.
In total, there are now probably more than a quarter of a million people displaced by the fighting, UN peacekeeping chief Herve Ladsous said, after briefing the UN Security Council on the crisis.
No current figures on a death toll were available, Ladsous added, but he estimated it was "very substantially in excess" of the 1000 deaths the UN reported just after conflict erupted on December 15.
The Security Council has approved sending an extra 5500 troops to South Sudan, who are only slowly arriving. Ladsous said it would need four to eight weeks to get the reinforcements in place.
Nepal is to provide 850 of the extra troops, UN officials said.
Both sides have been blamed for attacks on civilians, and Ladsous said the United Nations is not cooperating with the government Sudan People's Liberation Army (SPLA) or with Machar's forces.
The UN mission in South Sudan (UNMISS), which has been in place since the country's independence in July 2011, had provided logistical and other limited cooperation.
"Clearly in the present situation we are treating all sides equally. We are denouncing violations by whoever they are made. We are not cooperating with any side," said Ladsous.
"UNMISS military is focused 100 per cent on protecting civilians, and it will protect civilians against any armed force that threatens civilians including any members of the SPLA," UN peacekeeping spokesman Kieran Dwyer told AFP.
