A U.S. military aircraft came under fire Saturday during a reported evacuation mission in South Sudan, leaving four U.S. service members wounded, one critically.

“The aircraft diverted to an airfield outside the country and aborted the mission,” reads a statement from the U.S. military’s Africa Command.

The aircraft fell under attack as it headed to Bor, the capital of the state of Jonglei and ground zero of an escalating conflict between South Sudan’s military and defected armed forces.