World leaders have stepped up efforts to pull South Sudan back from the brink of all-out civil war, as fighting rages across the country including in a key oil-producing region.
Special envoys from the US and Nigeria were flying into the capital Juba, following on from a mission by foreign ministers from east Africa and the Horn and after an appeal for an end to the violence from United Nations chief Ban Ki-moon.
Fighting has gripped South Sudan for a week, after President Salva Kiir accused his former deputy, Riek Machar, of attempting a coup. Machar has denied this, and has accused Kiir of carrying out a vicious purge of his rivals.
The clashes have left hundreds dead and sent tens of thousands of people fleeing for protection in UN bases or to safer areas of the country.
The fighting has both ethnic and political dimensions, as troops loyal to Kiir, an ethnic Dinka, battle forces backing Machar, a Nuer, and the International Committee of the Red Cross (ICRC) said it was "alarmed" by reports of violence against civilians.
"We are extremely concerned about reports of both targeted killings of civilians and ill-treatment," said Melker Mabeck, head of ICRC's delegation in Juba. "Civilians must be protected from attack. Respect must be shown for human life and for the dignity of all people."
Foreign governments, including those of the US, Britain, Uganda, Kenya and Lebanon, have been organising special evacuation flights to pull out their nationals.
Canberra has urged Australians in South Sudan to leave while they still can.
Foreign Minister Julie Bishop said the situation was "exceptionally volatile", and recommended any Australians who had not yet left to do so as soon as possible.
The UN began evacuating "non-critical staff" from Juba to Uganda and will send military reinforcements to embattled towns further north, it said on Sunday.
US WILL TAKE ACTION 'IF NEEDED'
The United States has deployed about 46 additional troops to violence-wracked South Sudan and will take further action if necessary after they were attacked, President Barack Obama said Sunday.
On Saturday, unidentified gunmen shot at three CV-22 Osprey tiltrotor aircraft when approaching the rebel-held city of Bor, wounding four US servicemembers and forcing the military aircraft to abort a mission to evacuate Americans and instead head toward Uganda.
Obama said the troops were part of a unit of "approximately 46" servicemembers deployed Saturday to help evacuate Americans, amid growing concerns the country could slide into civil war.
"After the aircraft came under fire as they approached Bor, the operation was curtailed due to security considerations, and the aircraft and all military personnel onboard departed South Sudan without completing the evacuation," Obama wrote in a letter to Congress.
That contingent was in addition to another 45 troops sent this week to help protect US citizens, personnel and property at the US Embassy in the capital Juba.
"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," Obama wrote in a letter to Congress.
"I appreciate the support of the Congress in these actions."
Earlier, the State Department said Americans were safely evacuated from Bor to the capital Juba on UN and US civilian helicopters.
Around 380 US officials and private citizens, plus 300 citizens of other countries have so far been transported to Nairobi and other locations on four chartered flights and five military aircraft.
"Other US citizens may have left through other means. We strongly recommend US citizens in South Sudan depart immediately," State Department spokeswoman Jen Psaki said in a statement.
Obama has called for an end to the violence, warning the country was on the "precipice" of civil war and that any military coup would trigger an end to diplomatic and economic support from Washington and its allies.
