WASHINGTON - The United States on Sunday evacuated its citizens from Bor, the capital of South Sudan's Jonglei state engulfed in armed conflicts, after a failed attempt a day earlier.

American citizens and nationals from US partner nations were lifted from Bor to Juba, South Sudan's capital, on UN and US civilian helicopters, State Department spokeswoman Jennifer Psaki said in a statement.

She said steps were taken to ensure "fighting factions were aware these flights were a humanitarian mission."

Washington was forced to give up an evacuation effort on Saturday, after a US military aircraft sent in to airlift Americans came under fire with four soldiers wounded.

Psaki said some 380 US officials and citizens as well as about 300 citizens of other countries have been airlifted so far to Nairobi of Kenya and other places on four chartered flights and five military aircraft.

US President Barack Obama warned on Thursday that South Sudan, which won independence from Sudan in July 2011, "stands at the precipice" of sliding back into a civil war.

Fighting broke out in Juba on December 15 between forces loyal to President Salva Kiir and his former deputy Riek Machar, who was removed from office in July. Armed conflicts are spilling over to other parts of the country and setting off reportedly ethnic violence.

Obama deployed 45 US troops to South Sudan on Wednesday to protect American citizens and property.