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A UN official in South Sudan has spoken of an atmosphere of fear and desperation as violence escalates, the BBC reported.

Humanitarian Co-ordinator Toby Lanzer told the BBC about summary executions in Bor, in the restive state of Jonglei that has fallen to rebels.

He said that as well as people seeking refuge at the UN base there were many more hiding out in the bush.

Clashes broke out between troops loyal to President Salva Kiir and others backing his former deputy a week ago.

Meanwhile the US said it had evacuated its citizens from Bor.

Four US service personnel were wounded on Saturday when their aircraft were shot at, delaying an evacuation operation and prompting US President Barack Obama to consider further action.

"As I monitor the situation in South Sudan, I may take further action to support the security of US citizens, personnel and property, including our embassy, in South Sudan," he said in a letter to Congress leaders on Sunday.

Earlier the South Sudanese army confirmed that Bentiu, the capital of oil-rich Unity State, had also fallen to troops supporting former Vice-President Riek Machar.

"Bentiu is not in our hands," said military spokesman Philip Aguer.

President Salva Kiir has accused Mr Machar of attempting a coup.

Mr Lanzer, who spent several days in Bor, said the problem of people seeking refuge was growing daily.

"I'm quite concerned that in a few days' time we won't be talking about tens of thousands, we'll be talking about hundreds of thousands directly affected," he said.

"It's really very moving to see people just asking: 'Can you please keep me alive?'"

Mr Lanzer added that there was a danger not just from fighting by conventional armies but from groups of youths who he said were simply "out of control."

The government says it is trying to retake Bor, and the state has seen fierce fighting in recent days.

Two Indian peacekeepers and at least 11 civilians were killed in an attack on a UN compound in Akobo, Jonglei, on Thursday.