A long line of evacuees stretched from the departures area of Juba airport into the baking sun of South Sudan. Meanwhile, the dented doors of the arrivals building slowly disgorged dazed and exhausted oil workers in filthy overalls. They had fled to the capital from the further reaches of the world's youngest country, on the first leg of journeys home, bringing with them news of factional fighting that has taken the country to the brink of civil war.

Many of them such as Hassan Ali, a Pakistani engineer, with dark circles around his eyes, had just escaped from Unity state with terrifying accounts of killing in the vast oil fields that feed oil production in both Sudans.

For the last three days terrified oil workers have listened to the carnage beyond the perimeter.

"It was Sudanese killing Sudanese," he said. "They were killing each other with stones and knives," said one of his colleagues before making a cut-throat gesture. A third man, who gave his name as Dabeer, spoke of one attacker whom he had seen chopping the hands from victims. The men said that at least 16 people had died, many of them ordinary South Sudanese out of uniform. On their way to the airstrip in Unity town they had seen scores of scorched huts.

"There were more than 20 burnt to the ground," said Ali.

Unity state, which provides much of the crude on which South Sudan's fledgling economy relies, has become the centrepiece of a power struggle between government forces and an emerging armed opposition. A stream of defections by senior military officers has split the Sudan People's Liberation Army (SPLA), with Major General James Koang Choul, commander of the division that controls the state, emerging as the latest to turn his back on President Salva Kiir yesterday.

Heavy fighting between ethnic Dinka, who hail from the same tribe as the president, and the Nuer kinsmen of the fugitive former vice president Riek Machar have been struggling for control of the oil which is pumped by Chinese and Malaysian companies.

With a haze coming off the rough concrete blocks, outside Juba's squalid airport snaking lines of hundreds of Chinese oil workers tried to hide from the sun by putting coats and T-shirts over their heads. Song Liu had been waiting for nearly four hours for his company, the China Petroleum Engineering & Construction Corporation (CPECC) to get him out via Dubai to Beijing.

"There are some people who have died up there," he said of the fields in Unity. "We are all leaving."

All around him men and women in their khaki overalls with CPECC badges trudged slowly forward while foremen barked orders.

Nearby Saeed Mansour, an Indian freshly arrived in Juba from Unity state, spoke of emergency shutdowns at four oil facilities in the state while fighting raged around them.

One week on from a dispute in South Sudan's presidential guard that escalated into street fighting in the capital and accusations of a coup, thousands of foreign nationals have crowded the airport to escape with RAF flights and US airforce transport planes, while oil companies scramble to evacuate personnel.

Meanwhile, an attempted airlift by US forces from the restive state of Jonglei ended in their aircraft being fired upon by militia loyal to the rebel commander Peter Gadet.

US helicopters were en route to an evacuation site near the state capital of Bor when they came under fire with four American personnel wounded. "The aircraft diverted to an airfield outside the country and aborted the mission," the US Africa command said in a statement.

In the red dust outside the airport among the throng of soldiers, oil workers and South Sudanese waiting to leave Juba, James Marial Achak was still struggling to comprehend his own good fortune. He was one of the lucky few to catch the last flight out of Akobo, a tense outpost to the far east of South Sudan where a UN base was overrun on Friday, resulting in the deaths of two Indian peacekeepers. An ethnic Dinka, he was in the UN outpost just hours before it was stormed by a mob of two thousand Nuer youths, many of them armed.

"If I hadn't got on that plane I would have died," said the aid worker. "My children would have been orphaned."

His friend and fellow Dinka, Deng Majok was not so fortunate. When the crowd swarmed around the small base firing weapons, the Indian peacekeepers were forced to let them in.

The UN soldiers were then tricked into gathering nearly two dozen Dinkas, mainly local government officials into one place with assurances that they would be protected from the mob. Once in the same spot several men opened fire on the peacekeepers, killing two of them and hitting a third in the chest. Then they massacred the Dinka. Majok survived by covering himself with the corpses of the others and not responding when one of the crowd trod on him. He was later smuggled out of Akobo as far as Malakal, in Upper Nile state.

"There was nothing the peacekeepers could do," said Marial Achak, who had reached his friend by phone on Friday night. "The Nuer were so many and they were so few."

In Juba itself a form of calm and normality has returned to some parts of the city. Shops have reopened and civilian traffic has returned to the streets where only a few days ago battles raged between different army factions. But there are signs everywhere of what has passed. A battle tank is parked next to the entrance to the president's residence and a trio of pick up trucks mounted with anti-aircraft guns are parked on the corner.

Only a few hundred metres away, the residence of President Kiir's rival, Riek Machar, is pockmarked with craters from heavy calibre bullets and a corner wall has been demolished by artillery.

"This calmness is ambiguous," said Patrick Kapukha, a Kenyan aid worker with four years' experience in the country. "A cycle of revenge and fear is building momentum."

He said that his staff were relaying reports of large-scale mobilisation among different ethnic communities intent on revenge for what they see as targeted killings.

"The leaders have got to come together and do so on television or this is going to get much worse," he warned.