cThe 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.