British, Canadian and Kenyan citizens are among 3,000 foreigners trapped in a South Sudan city experiencing bouts of heavy machine gun fire.
They are in one of the most violent areas of a week-long conflict that has probably killed more than 1,000 people, a top UN official said.
Australians, Ugandans and Ethiopians are also among 17,000 people seeking protection at a UN base in Bor, a city that could see increased violence in coming days, said Toby Lanzer, the UN's humanitarian co-ordinator.
The death toll from a week of violence in South Sudan has probably passed 1,000 people, though there are no firm numbers available, he said.
The number of internal refugees is probably more than 100,000, said Mr Lanzer, who is seeking urgent financial assistance from the US, Britain and other European countries.
"I know there are many thousands of people seeking protection in churches," he said.
"I know that we have our own staff that have literally walked into the bush and are communicating from there. That's where they say they are safest."
UN Secretary-General Ban Ki-moon urged the Security Council late last night to add 5,500 troops and police to the 7,000-strong UN peacekeeping mission in South Sudan.
He cited growing violence in many parts of the country, human rights abuses, "and killings fuelled by ethnic tensions".
He proposed the troops be transferred from UN missions in Congo, Darfur, Abyei, Ivory Coast and Liberia, along with three attack helicopters, three utility helicopters and a C130 military transport plane.
After an emergency Security Council meeting, France's UN Ambassador Gerard Araud, the current council president, said the council will vote on a US-drafted resolution authorising the transfers this evening.
He said there was "a positive reaction" from all 15 council members.
The secretary-general called on member states urgently to provide transport to get the troops, police and equipment to South Sudan.
He said the UN mission's capacity to investigate human rights abuses is also being urgently strengthened.
Samantha Power, the US ambassador to the UN, said: "The future of South Sudan is in jeopardy.
"The leaders of South Sudan face a stark choice. They can return to the political dialogue and spirit of co-operation that helped establish South Sudan or they can destroy those hard-fought gains and tear apart their newborn nation."
The violence began on December 15. South Sudan President Salva Kiir, an ethnic Dinka, has said an attempted military coup triggered the violence, and the blame was placed on ousted former vice president Riek Machar, an ethnic Nuer.
Other officials have since said a fight between Dinka and Nuer presidential guards triggered the fighting.
South Sudan experienced decades of war with Sudan, which it peacefully broke away from in 2011.
The US over the last week has evacuated 380 Americans and 300 others from South Sudan, which has seen vicious, ethnically targeted violence spread through the nation.
Meanwhile the UN has discovered a mass grave containing about 75 bodies in Bentiu in the country's oil rich Unity State.
At least two other mass graves are reported to have been found in Juba.
A spokeswoman for the Geneva-based human rights office says the bodies in Bentiu reportedly belonged to ethnic Dinka who were members of the Sudan People's Liberation Army.