Updated at 3:45 p.m. ET.

U.S. military officials attempting to evacuate Americans in a conflict-riddled region of South Sudan were shot at by rebel gunfire, injuring four and forcing an aborted mission.

The South Sudanese government said it lost control of Bor, the capital of the state of Jonglei, earlier this week, to renegade troops, and was attempting to retake the state capital. Fighting reportedly began Saturday, after rebel soldiers randomly opened fire on civilians, the Associated Press reports. Three CV-22 Ospreys, aircrafts that can fly like a helicopter and a plane, were forced to divert to an airfield outside the country.

A United Nations helicopter helping to evacuate peacekeepers and civilians was also struck in Jonglei Saturday.

President Barack Obama urged peaceful negotiations to resolve the political conflict in South Sudan and threatened to cut off U.S. support if military force is used to seize power, according to a White House statement. The president also emphasized that the nation’s leaders are accountable for securing the safety of American citizens amid the clash.

Tensions are mounting over a potential civil war in the newly formed country, after South Sudan President Salva Kiir ousted vice president Riek Machar and accused him of a failed coup. Ethnic tensions are being blamed. The region has been the backdrop to some of the worst violence the country has seen since it became independent from Sudan in 2011.

Machar said rebel troops have seized the oil-rich state Unity and now control majority of the country, the BBC reports. The former vice president confirmed with the BBC that the rebel troops were under his power.

[AP]