The group arrived in Djibouti on the Horn of Africa after having been ordered from their base in Spain, and were primed to travel to South Sudan if required, the US Africa Command said.
The move came as the United Nations was expected Tuesday to vote to nearly double the size of its peacekeeping force in South Sudan, and as the UN's top human rights officer described new evidence of atrocities and extrajudicial killings, including the discovery of mass graves.
The central African country, which became independent from Sudan less than three years ago, has descended into vicious warfare between fighters loyal to the president and those allied to the former vice-president. Hundreds have died and tens of thousands have fled their homes.
Evacuation flights arranged by the Pentagon have flown more than 300 US citizens from the capital, Juba, in the last week. Four American troops were injured last weekend when their aircraft came under fire.
"The group of 150 people which is now in Djibouti has been sent to be better positioned quickly if the security situation in South Sudan demands it,” an AFRICOM spokesman told the Monitor.
“They are primarily intended to assist to protect US citizens or property, and there I’m talking about the embassy, should the State Department or any other arm of government request it.”
In his strongest statement yet on the 10-day-old crisis, UN Secretary-General Ban Ki-moon demanded an end to the violence and said crimes against humanity may already have taken place.
“Those responsible at the senior level will be held personally accountable and face the consequences,” Mr. Ban told reporters ahead of emergency Security Council talks on the crisis.
He asked the Council to approve a surge of 5,500 new peacekeepers to bolster the 7,000-strong force already in South Sudan.
Distant prospect for talks
Officially, 81,000 people have fled their homes and 500 have died since soldiers loyal to the former vice-president mutinied on Dec. 15 and began a firefight in an army barracks in Juba, the capital.
For its first two days, the fighting initially affected only that city, on the banks of the White Nile. Since then it has spread across half of South Sudan’s 10 federal states, including those where the country’s oil fields lie.
There are fears that rebel forces could seize control of those supplies, allowing them effectively to hold the government to ransom and giving them a significantly better hand should peace talks take place.
Talks look to be a distant prospect, however. Riek Machar, the former vice-president who heads the insurrectionists, said he would negotiate only if his allies were freed from jail, where they have been held since the fighting flared.
Salva Kiir, South Sudan’s embattled president, would not agree to those “preconditions” to talks, his government spokesman said Monday, leaving the political situation deadlocked.
That death toll of 500 is widely believed to be a major under-estimation. Witnesses told the BBC Monday of an incident where 200 were killed in one Juba police station. One of the 12 men who escaped described how he survived by hiding under dead bodies.
Navi Pillay, the United Nations High Commissioner for Human Rights, said Tuesday that at least one mass grave had been discovered in South Sudan. She said the atrocities had included "mass extrajudicial killings, the targeting of individuals on the basis of their ethnicity and arbitrary detentions." She added that in addition to a mass grave found in Bentiu, in Unity State, her office had received reports of two or more others in the capital.
Toby Lanzer, the UN’s humanitarian coordinator in the country, told the BBC he had witnessed “some of the most horrible things that one can imagine” during a visit to Bor, a city north of Juba that fell to the rebels last week.
The violence pits fighters from Mr. Machar’s Nuer people against Mr Kiir’s Dinkas people, the two largest tribes in the country. But the motivations for the violence appear more political than tribal.
The rift between the two men worsened in July when Kiir fired Machar and several of his ministerial allies. Since then, Machar has said that the president is turning increasingly dictatorial and has promised to stand against him in elections due in 2015.
On Dec. 22 US President Barack Obama warned Congress that he “may take further action” if the situation deteriorates. Donald Booth, Mr. Obama’s envoy for Sudan and South Sudan, arrived in the capital Monday to meet the president and opposition representatives.
He is one of many international envoys who have begun jetting in to Juba to try – so far in vain – to encourage an end to the fighting and to bring the two men to the negotiating table.
Meanwhile, thousands of expatriates remain behind despite the US, Britain, Canada and other nations laying on emergency evacuation flights for their citizens.
Many have significant commercial investments that they want to protect. They had not expected there would be major new bouts of violence in the capital.
“It’s a calculated risk that Juba will remain unaffected by fighting and we can stay and keep business ticking over,” said Mark Dilley, a British logistician working for a private contractor in South Sudan.
“There was a temptation to get on those evac flights. There’s pretty much no-one left at the British Embassy to help if things get ugly here, but I think those of us left reckon we can manage.”