Three U.S. military aircraft took gunfire during an evacuation mission in violence-torn South Sudan today, resulting in injuries to four service members.

As the three CV-22 Osprey tilt-rotor aircraft approached the town of Bor, they "were fired on by small arms fire by unknown forces, according to U.S. Africa Command. All three aircraft sustained damage during the engagement." CV-22 tilt rotor Ospreys are the version of the aircraft flown by U.S. Air Force Special Operations forces. The aircraft can take off like a helicopter and fly like a plane once airborne.

After the Ospreys took fire, the evacuation mission was aborted and the aircraft "diverted to Entebbe, Uganda, where the wounded were transferred onboard a U.S. Air Force C-17 and flown to Nairobi, Kenya for medical treatment."

The four service members aboard the aircraft who were injured in the gunfire were said to be in stable condition.

The U.S. evacuation mission to Bor, a city in eastern South Sudan that has also been wracked by the violence that has gripped the region this week, was presumably to take out American relief workers based in the city.

"This morning's mission was aborted when the aircraft was fired on," State Department spokeswoman Beth Gosselin said. "We continue efforts to evacuate US Citizens from Bor. For their safety we will not provide details on the numbers of evacuees, their location or possible evacuation plans."

Violence broke out earlier this week between rival factions of South Sudan's military, and there is concern that the violence could become a full-blown civil war.

Hundreds have been killed in the fighting which has mainly taken place in the capital city of Juba. Bor, in the eastern state of Jonglei, has also seen heavy violence and is reportedly controlled by the military faction not loyal to the government.

On Monday, two U.S. military C-130 aircraft evacuated 120 American and western diplomats from Juba after the State Department requested U.S. military assistance in evacuating non-emergency personnel.

The planes also carried in 45 members of the U.S. military's East Africa Response Force (EARF) to stay behind in Juba to protect the U.S. embassy and the American diplomats who remained behind.

The EARF is one of two US military quick response forces for Africa established after the Benghazi consulate attack. The response force of 500 Marines based in Spain can respond to crises in North and West Africa, the smaller 100 man EARF is based in Djibouti and can respond to emergency situations in eastern Africa.

Just this week the U.S. military was wrapping up an airlift mission to the neighboring Central African Republic that is also in the middle of a violent crisis. For the last two weeks two U.S. Air Force C-17 aircraft had transported more than 800 Burundian troops and their equipment to assist with peacekeeping efforts there.

South Sudan is the world's newest country after breaking away from Sudan in 2011.