UN Secretary General Ban Ki-moon has asked the UN Security Council to send 5,500 more peacekeepers to South Sudan.
Mr Ban made the recommendation for the two-thirds increase in the size of the force in a letter to the 15-member council, in which he also called for 423 more police officers.
There are currently around 6,700 UN troops and 670 police officers in the UN force in South Sudan, which is known as UNMISS.
The Security Council met tonight to discuss the situation in South Suda and is likely to adopt ar esolution approving the increase in peacekeepers tomorrow, council diplomats said.
"The situation is obviously urgent and the Security Council will respond urgently. If it's necessary to take decisions, then we will take decisions by tomorrow," British UNAmbassador Mark Lyall Grant said.
Diplomats said US Ambassador Samantha Power distributed the draft resolution to the council, adding that Ms Power said she hoped it would be adopted by noon tomorrow.
Mr Ban said the additional troops would be drawn from other nearby UN and African Union missions, such as those in Democratic Republic of Congo, the Sudanese regions of Darfur and Abyei, Liberia and Ivory Coast.
He said five infantry battalions, three attack helicopters, three utility helicopters, one C130 military transport aircraft and three police units should be transferred to South Sudan.
"I would be grateful if the Security Council would approve the transfer of the relevant personnel and assets to UNMISS on an urgent basis in order to help ensure the protection of civilians and the protection of United Nations personnel and assets," Mr Ban wrote in his letter to the council.
He said the United Nations was obtaining the consent of the troop and police contributing countries and coordinating with the "peacekeeping operations concerned to ensure that the timing and duration of this proposed temporary re-deployment does not prejudice the implementation of their respective mandates."
Machar calls for release of political allies
Riek Machar, the former vice president of South Sudan leading a rebellion against the government, has said he was ready for dialogue to end the conflict but said President Salva Kiir must first release his detained political allies.
Mr Machar said he had spoken to Ethiopian Foreign Minister Tedros Adhanom, leader of a team of African mediators trying to end more than a week of fighting that has killed hundreds of people and driven thousands from their homes.
Mr Ban said around 45,000civilians were seeking protection at UN bases in South Sudan.
"The world is watching all sides in South Sudan. Attacks on civilians and the UN peacekeepers deployed to protect them must cease," he said.
UNMISS evacuates personnel from Bor compound
Earlier, UNMISS evacuated its non-essential staff and wounded civilians from the town of Bor, as heavy fighting between government forces and rebels continues.
UNMISS says 15,000 civilians are currently sheltered within the UN compound in Bor.
An estimated 20,000 people have crossed to the adjacent state, fleeing the violence.
UNMISS said a further 5,000 civilians have sought shelter and protection at the UNMISS compound in Bentui, the regional capital of oil-rich Unity State.
Yesterday, three US aircraft came under fire from unidentified forces while trying to evacuate Americans from the spiralling conflict.
The US military said four of its members were wounded in the attacks.
The aircraft came under fire while approaching the evacuation site, the military's Africa Command said in a statement.
It has deployed around 150 marines to a base in the Horn of Africa to prepare for possible further evacuations of US citizens from the deepening conflict in South Sudan.
The Marines were sent to a base in Djibouti, a move that would allow them to deploy to South Sudan more quickly, if asked.
South Sudan's government said it would start a major offensive to retake two strategic towns controlled by rebels loyal to Mr Machar.
Earlier, South Sudan's President Salva Kiir said he is committed to beginning talks with Mr Machar, a senior US official said after meeting Mr Kiir in the capital Juba
It was not clear how soon a meeting could be arranged.
US special envoy Donald Booth was sent to South Sudan at the weekend to seek a diplomatic solution to the violence.