A CV-22 Osprey aircraft, like those damaged on Saturday, Dec. 21, 2013 while trying to evacuate American citizens in Bor, the capital of South Sudan's Jonglei state. (AP Photo/US Air Force, Master Sgt. Jeremy T. Lock, File)

(CNSNews.com) – President Obama informed Congress Sunday that if necessary he “may take further action” to protect U.S. citizens, personnel and property, including the embassy, in South Sudan, where troops deployed to evacuate Americans came under fire on Saturday.

“This action has been directed consistent with my responsibility to protect U.S. citizens both at home and abroad, and in furtherance of U.S. national security and foreign policy interests,” he said in a formal letter to congressional leaders. Obama is currently on vacation in Hawaii.

The world’s youngest independent country has been in turmoil since what its government called a coup attempt on December 14 by forces loyal to a former vice-president, Riek Machar, who was dismissed along with the entire cabinet in July.

On Thursday the Machar faction claimed to have seized control of Bor, the capital of Jonglei state. Two U.N. peacekeepers and 11 local civilians were killed in an attack on a U.N. base in the state, and when the U.N. sent helicopters to evacuate peacekeepers one was forced to make an emergency landing after coming under small-arms fire.

On Saturday, three U.S. military CV-22 tilt-rotor Ospreys were damaged after coming under small-arms fire as they approached Bor. According to U.S. Africa Command, four personnel were injured, flown to Nairobi for medical treatment, and were reported to be in a stable condition.

In his letter, Obama said about 46 U.S. troops had been deployed to help evacuate Americans from Bor.

“After they came under fire, he said, “the operation was curtailed due to security considerations, and the aircraft and all military personnel onboard departed South Sudan without completing the evacuation.”

“As I monitor the situation in South Sudan, I may take further action to support the security of U.S. citizens, personnel, and property, including our embassy, in South Sudan,” the president wrote.

The military attributed the shooting at the Ospreys to unknown forces, although South Sudan’s government pointed out that area was in the hands of the rebel troops.

A U.S. soldier assists a passenger in checking off his name on the flight’s manifest before take-off during emergency evacuation flights from Juba, South Sudan. (Photo: U.S. Embassy, Juba)

Later, the State Department reported that U.N. and U.S. civilian helicopters had managed to fly American citizens and citizens of other countries from Bor to the national capital, Juba, on Sunday morning.

Four chartered flights and five military aircraft have now evacuated a total of some 380 American officials and citizens and about 300 citizens of other countries, from South Sudan, State Department spokeswoman Jen Psaki said.

Last week the U.S. deployed about 45 troops to protect the embassy in Juba, where Defense Secretary Chuck Hagel said during a Pentagon briefing on Thursday they were “working very closely” with embassy security.

Chairman of the Joint Chiefs of Staff Army Gen. Martin Dempsey told the same briefing that since the Sept. 2012 terror attack on the U.S. Consulate in Benghazi, “we’ve put response forces at various levels of readiness that we dial up and dial back as the situation requires.”

The troops deployed in South Sudan are from the Djibouti-based East Africa Response Force, created after the Benghazi attack. It falls under the Combined Joint Task Force – Horn of Africa, and comprises troops rotating from the Fort Riley, Kan.-based 1st Infantry Division.

In a deepening of the crisis, which has already claimed hundreds of lives, South Sudan’s government said Sunday renegade troops had seized control of Bentiu, the capital of oil-rich Unity state.

Oil is the main revenue-earner of the country, which became independent in July 2011 after a referendum favored secession from Sudan.

Earlier, a 2005 peace agreement reached with significant American involvement ended a two decade-long civil war between the Muslim north and the mostly Christian and animist south. An estimated two million people died during the conflict.