UN chief Ban Ki-moon is likely to ask the Security Council to send more peacekeepers to South Sudan to protect civilians from worsening violence.
Clashes which started in the capital Juba on 15 December have reached the country's vital oil fields and destabilised the state that won independence from Sudan only in 2011.
Thousands have sought refuge at a UN compound in Bor. Photo: AFP / UNMISS / Rolla Hinedi
Hundreds of people have been killed, with reports of summary executions and ethnically-targeted killings.
UN diplomats told Reuters the 15-member council is likely to approve about 5,000 more troops and 280 more police for the UNMISS peacekeeping mission.
At present there are 6,800 peacekeeping troops and nearly 700 police in South Sudan.
South Sudan's government has said it plans a major offensive to retake two strategic towns controlled by rebels loyal to former vice-president Riek Machar.
Information Minister Michael Makuei said the government continued to control parts of oil-rich Unity State, even after the army divisional commander there John Koang defected and joined Mr Machar.
Mr Makuei said troops were preparing to launch an offensive against rebel groups in the state's capital Bentiu, and denied reports that oil production has been disrupted in Unity State.
The government also planned s to attack and retake Bor, the capital of Jonglei State.
The United Nations said on Monday there was sporadic fighting in Bor, which fell to rebels on Wednesday.
UN spokesman in South Sudan Joe Contreras said about 17,000 people were seeking refuge at a UN compound.
A New Zealand nurse working with the International Red Cross in the South Sudanese capital Juba said medical services are inundated with wounded victims of rebel and Government violence.
Felicity Gapes told Radio New Zealand's Morning Report programme staff are seeing many people with gunshot wounds, and refugees who have fled without food, water, shelter or medical supplies.
