Rebels hold key oil capital in South Sudan

On Saturday gunfire hit three U.S. military aircraft trying to evacuate American citizens in Bor, wounding four U.S. service members in the same region gunfire downed a U.N. helicopter on Friday.

It remains unclear how many U.S. citizens are still stranded in Bor and other rural towns.

Earlier this week the top military general in Bor defected with his troops, starting a rebellion that appears to be spreading to other parts of the country.

Aguer said Bor is still under the control of pro-Machar forces, disputing reports the rebels had fled as government troops advanced on Bor.

South Sudan's President Salva Kiir, an ethnic Dinka, said on Monday that an attempted military coup had triggered the violence, and the blame was placed on Machar, an ethnic Nuer.

But officials have since said a fight between Dinka and Nuer members of the presidential guard triggered the fighting that later spread across the East African country.