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.

The spectre of a wider war came as officials confirmed that rebels loyal to former vice president Riek Machar, who President Salva Kiir accused of masterminding the Sunday coup attempt, seized the Jonglei state capital Bor, killing and displacing civilians.

Forces commanded by former divisional commander, Peter Gatdet, who switched his loyalty to Mr Riek on Wednesday, stormed and seized the capital last evening, said Bor town Mayor Nhial Majak.

"They are in control of Bor. People are now running to Athooch," he told Africa Review, referring to a settlement north of Bor town.

"This morning, they continued shooting," he said.

Defence Minister Kuol Manyang Juuk confirmed to local media that the military had lost control of the state capital.

Hundreds of people have been killed and thousands more terrified civilians have fled their homes since the fighting broke out in Juba 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 that as fighting occurred in Bor, tensions were rising in other states.

Other reports said the fighting was spreading to Torit, the capital of southeastern Eastern Equatorial State.

Barricaded homes

President Kiir has blamed the bloodshed on an attempted coup bid by his arch-rival 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.

The fighting has highlighted the bitter fault-lines 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.

Kenya Airways was Thursday morning scheduled to make its first flights to Juba since the violence broke out, while earlier a large batch of Kenyans and Ugandans were evacuated out of the country in 13 buses.

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