UN human rights experts on a fact-finding mission to South Sudan warned Wednesday of an escalation in ethnic violence in the war-torn country.

"Many expect intensified fighting now that the dry season is setting in," said Yasmin Sooka of the UN Commission on Human Rights in South Sudan.

Sooka was speaking to the press in the capital Juba at the end of a 10-day visit during which the three-member team spoke with civilians in the battleground towns of Bentiu, Malakal and Wau, as well as government officials and members of civil society.

"There are unprecedented levels of violence and ethnic tension all over South Sudan," Sooka said.

"Any sense of national identity is crumbling and tribal or ethnic identity is taking over. I repeatedly heard of the desire for revenge," she added.

South Sudan's current conflict began nearly three years ago when President Salva Kiir accused his former deputy and political rival, Riek Machar, of plotting a coup. Since then the world's newest nation has fractured along ethnic lines in a civil war characterised by atrocities.

Sooka said government and rebel armies were both forcibly recruiting soldiers — including children — and warned that "renewed recruitment is an indicator that all the parties are preparing for the next conflict".

The UN rights experts repeated calls for an arms embargo, sanctions, the deployment of another 4,000 peacekeepers and the establishment of a special war crimes court.

The US on Wednesday also warned of escalating violence.

"We have credible information that the South Sudanese government is currently targeting civilians in Central Equatoria and preparing for largescale attacks in the coming days or weeks," Keith Harper, the US representative at the UN Human Rights Council, said in Geneva.

"In the last two weeks, the government has mobilised at least 4,000 militia from other areas of South Sudan and is staging these fighters in Equatoria to begin conducting attacks," Harper said.

Earlier this month the UN's Special Advisor on the Prevention of Genocide, Adama Dieng, told the Security Council there was a risk of "outright ethnic war" and the "potential for genocide".

The UN rights experts are expected to publish a report of their findings in March.