The United Nations on Sunday reported armed groups close to a key town in South Sudan amid battles between supporters of the country's president and vice president.
A UN surveillance aircraft spotted the groups 50 kilometers (30 miles) from Bor, capital of Jonglei state, which has become a focus of a power struggle between President Salva Kiir and former vice president Riek Machar.
More than 1,000 people have died since fighting between the two erupted on 15 December, and a UN statement said the number of people who have taken refuge in its bases around the country has grown to 75,000.
Fears of heightened conflict have grown amid reports an army of Machar's youth followers was gathering around Bor.
The UN Mission in South Sudan (UNMISS) has been tracking reports of the armed groups' movements but cannot confirm the size or location of Machar's followers, said the UN statement.
“UNMISS today conducted aerial reconnaissance and reports that they have identified some armed groups approximately 50 kilometers northeast of Bor,” said the statement.
However, it did not say who the groups were.
UNMISS chief Hilde Johnson, who is to brief the UN Security Council on Monday on the South Sudan crisis, has been seeking to persuade political and community leaders to head off the advance on Bor, the statement added.
The UN says the number displaced by the conflict has grown to 180,000 people and up to 75,000 have sought refuge in UNMISS bases in Juba, Bor, Bentiu, Malakal and Pariang.
There have been battles between Kiir and Machar forces for control of Bentiu and Malakal which are in key oil regions.
The UN said that Bentiu, the main city in Unity state, was “calm but tense” but it added that “there have been reports of heavy fighting in Mayom town” which is just west of Bentiu.
Juba has claimed that Machar, the de facto leader of rebels fighting the government, has recruited up to 25,000 young fighters from his Nuer tribe in the eastern state of Jonglei and that they are “ready to attack any time”.
According to Jonglei's acting governor Ogato Chan, the Nuer fighters were around 110 km (70 miles) from the state capital of Bor, currently under the control of government forces.
The Nuer fighters are members of a tribal militia known as the “White Army” that became synonymous with years of violence and terror during the 1990s civil war.
Experts say the name comes in part from the ash the fighters use to protect their skin from insects.
In 2011 and 2012 the White Army turned on the Murle ethnic group, killing hundreds in a conflict over cattle theft.
“These youth have been reportedly moving across the state for some time now with the possible intention of attacking other communities,” UNMISS spokesman Joseph Contreras said.
Claims that the White Army was being mobilised have cast a shadow over peace talks spearheaded by regional leaders to end two weeks of bloody violence believed to have killed thousands.