WASHINGTON — Three United States aircraft flying into a heavily contested region of South Sudan to evacuate American citizens were attacked on Saturday morning and forced to turn back without completing the mission, American officials said. Four service members were wounded, one seriously.

The United States had been evacuating Americans from the country, where a political crisis exploded in violence last week, for several days, but the mission on Saturday was the first into rebel-controlled territory.

The Special Operations forces took off from Djibouti heading for Bor, the capital of Jonglei State, where some 14,000 refugees were holed up in a United Nations compound surrounded by armed young men, American officials said.

As the aircraft, tilt-rotor CV-22 Ospreys, which can fly like an airplane and land like a helicopter, approached Bor around 10 a.m. local time, they “were fired on by small-arms fire by unknown forces,” the military said in a statement.