GENEVA (AFP) -- The number of South Sudanese fleeing their violence-wracked nation for severely overcrowded refugee camps in neighbouring countries could exceed 100,000 by the end of the month, the UN said Friday.

More than 86,000 South Sudanese have already flooded across the country's borders since the brutal conflict erupted on December 15, when President Salva Kiir accused his former deputy Riek Machar of plotting a coup.

"With people still arriving at a rate of around 1,000 a day, we are looking at the prospect of refugee numbers exceeding 100,000 by the end of January," Adrian Edwards, spokesman for the UN refugee agency UNHCR, told reporters in Geneva.

As many as 10,000 people are believed to have been killed in the fighting pitting forces loyal to Kiir against a loose coalition of army defectors and ethnic militia nominally headed by Machar, and the United Nations has accused both sides of carrying out atrocities.

In addition to the tens of thousands fleeing across borders, some 468,000 South Sudanese have been displaced inside the world's newest country, according to the UN's humanitarian agency.

Uganda has taken in the largest number of South Sudanese refugees, around 46,600 so far -- most of them under the age of 18.

Many find themselves crammed into the overcrowded Dzaipi transit centre in the north, Edwards said, pointing out that the centre designed to hold 400 people was currently lodging more than 20,000 refugees.

"People are contending with issues relating to hygiene, food and lack of water," he said.

UNHCR was racing to transfer some 500 families each day from Dzaipi to the nearby settlement of Nyumanzi, and had created a second transit centre that could hold 4,000 people.

He said water appeared to be the most urgent challenge, although health and shelter were also problematic since many people were sleeping out in the open.

Some refugees reported waiting up to four days to get water, he said, "while others are sleeping in line at water pumps with their jerry cans."

More than 20,600 South Sudanese refugees in Ethiopia and at least 8,900 in Kenya were also facing challenges linked to water, health, sanitation, shelter and education, Edwards said.

"In Kenya in particular, we are seeing many children separated from their parents," he said.

An estimated 10,000 South Sudanese have also crossed into Sudan, the states of South and West Kordofan, Edwards added, stressing however that the Khartoum government only considers some 2,000 of them to be refugees.