An official in Bor, north of the capital, Juba, said soldiers attacked each other at two military barracks and one journalist said troops loyal to Vice-President Riek Machar now controlled the barracks, suggesting violence was increasingly running along ethnic lines.

Soldiers in Juba have clashed since Sunday, killing up to 500 people in what President Salva Kiir said was an attempted coup launched by Mr Machar’s supporters. According to the online journal Sudan Tribune, Mr Machar denied any role in the fighting or any coup attempt.

Juba was calm after sporadic overnight gunfire but fighting was also reported to have flared in Torit, east of Juba, adding fresh instability to a volatile region.

"The worry is that once this conflict spreads out of Juba to other areas it is in (a) much more ethnic landscape, and then you have the remobilisation of the old militias," said one western diplomat who has long experience of South Sudan.

The president fired Mr Machar in July and political tensions have simmered since then in the oil-producing nation, which relies on crude exports by means of a pipeline through Sudan.

Until now, fighting had appeared to stay away from the oilfields.

The government said it had arrested 10 people, including seven former ministers, over the "foiled coup" and wanted to question several others, including Mr Machar.

In Bor, where ethnic Nuer soldiers loyal to Mr Machar massacred hundreds of Dinka — the president’s ethnic group — in 1991, the locals feared that the fighting could spill beyond the barracks.

A broader conflict could threaten vital aid and be exploited by neighbouring Sudan, which has had persistent rows with Juba over their undefined borders, oil and security.

That would further hurt efforts to build a functioning state in the south. "Last night, there was fighting in two military barracks," said the deputy governor of Jonglei state, Hussein Maar, although he said the town of Bor was calm.

A journalist in Bor told Reuters by telephone that troops led by commander Peter Gadet, an ally of Mr Machar, took control of bases that were abandoned by outnumbered Dinka soldiers.

But details were sketchy, and, according to another account, only one base was in the hands of Mr Machar’s allies. The United Nations (UN) in South Sudan reported fighting yesterday morning in the Bor area, saying on Twitter that more than 1,000 civilians had sought refuge in the UN compound.

It also reported clashes at a military base in Torit.

The diplomat said the expanded fighting was tipping the nation towards an ethnic civil conflict that was "difficult to roll back", saying Mr Kiir had raised the stakes by branding the clashes a coup attempt rather than just infighting in the army.

"It will impact a lot of countries, and they are not beacons of stability," he said of the region around South Sudan. "There will be negative consequences for everybody."

Traffic returned to Juba’s roads and its airport reopened amid a tense calm, in the capital of a nation the size of France with 11-million people but barely any tarmac roads.

The US state department said it was organising evacuation flights and the UK said it was evacuating some embassy staff and gathering names of other Britons who wanted to leave. Many aid workers live and work in Juba.

Diplomats said the UN had reports of 400 to 500 people killed and up to 800 wounded in the nation that declared independence in 2011 from Sudan.

"Most people are scared they might be confronted with a mob or see dead bodies," said one aid worker in Juba. The city was calmer yesterday morning, after residents awoke to heavy gunfire and artillery blasts on Monday and Tuesday.

Political tensions have been mounting since Mr Machar’s dismissal. The former vice-president has said he would run for president and has accused Mr Kiir of being dictatorial. Mr Kiir had said before the clashes that his rivals were reviving rifts that provoked infighting in the 1990s.

Mr Kiir has faced mounting public criticism for doing little to improve life in one of Africa’s poorest nations.

"Salva (Kiir) must recognise that the charge of his being ‘dictatorial’ has taken deep hold, and he must do what is necessary to shed the label as much as possible," South Sudan expert Eric Reeves wrote in an assessment of the violence.

Reuters