The United Nations is warning that violence was spreading in South Sudan, as fierce fighting in the world's youngest nation prompted the US to evacuate Americans and other foreigners.

There were fears in the international community that the poor and unstable nation, which became independent from Sudan in 2011, could slide back into civil war.

Hundreds of people have been killed and thousands more terrified civilians have fled their homes since the fighting broke out on Sunday to seek protection at UN bases.

South Sudan's Red Cross reported at least 19 civilians killed in new clashes between rival army factions that have now spread outside the capital Juba, UN spokesman Martin Nesirky told journalists in New York on Wednesday.

He said the fighting occurred in Bor, capital of eastern Jonglei state, and that tensions were rising in other states.

South Sudan President Salva Kiir has blamed the bloodshed on an attempted coup bid by his arch-rival, former vice president Riek Machar.

Kiir said he was ready to "sit down" with Machar to try to solve the crisis.

But Machar, who was sacked by the president in July and is now a fugitive, denied any coup attempt.

"Kiir wanted to use the alleged coup attempt in order to get rid of us," Machar told the Paris-based Sudan Tribune website in his first public remarks since the fighting erupted.

UN peacekeeping chief Herve Ladsous told the UN Security Council that between 400 and 500 bodies had been reportedly taken to hospitals in Juba since the fighting broke out, while another 800 people had been wounded.

"There is a heavy toll, it is obvious," UN Security Council President Gerard Araud said, without confirming how many people had died.

In Juba, early shooting gave way to a lull later on. Many residents have spent the past two days barricaded in their homes, too afraid to move.

Others used pauses in the sporadic and often intense battles to flee to safer areas, including UN bases, despite Kiir's pleas that they return to their homes.

Fighting along ethnic lines

The fighting has highlighted the bitter faultlines in the country, which is awash with guns.

Kiir and Machar hail from different ethnic groups and fought on different sides during Sudan's 1983-2005 civil war.

The United States used two C-130 military transport planes and a charter aircraft to fly 150 Americans as well as US and foreign diplomats out of the country, the State Department said.

"The security situation was getting ugly. There was shooting at the airport," a US Defence Department official said on condition of anonymity.

Further flights could follow after Washington urged all Americans to depart the country immediately.

Britain also announced some of its embassy staff were temporarily leaving the country.

Several regional airlines resumed flights at Juba's airport, with long lines of aid workers and expatriates boarding the first flight they could out of the country. Others left by bus for Uganda.

"Sad/conflicted about leaving place I love in turmoil," wrote American aid worker Erin Polich on Twitter after arriving in Kenya.

The South Sudanese government said 10 key figures, many of them former ministers, have been arrested in a crackdown on alleged coup participants.

Kiir said a powerful military commander, Peter Gadet -- who rebelled in 2011 but then rejoined the army -- had mutinied again, launching attacks in Jonglei in support of Machar.

Fighting was reported late Tuesday in the state capital Bor, with shooting breaking out there again in the early hours of Wednesday.

"Hundreds of civilians have been streaming into our camp on the outskirts of the town, they're now over the 1,000 mark, and Bor is very tense," said UN Mission in South Sudan spokesman Joe Contreras.

The UN also reported clashes in the town of Torit, state capital of Eastern Equatoria.

Peacekeeping chief Ladsous said between 15,000 and 20,000 people had sought UN protection in Juba.

Medical teams in Juba's two main hospitals were having trouble coping, according to the International Committee of the Red Cross.