American citizens were evacuated from the South Sudanese city of Bor on Sunday, a day after US troops were injured by rebels when their aircraft was fired upon during the rescue.

Officials said that an unspecified number of Americans and other foreign nationals were airlifted from Bor to the capital Juba safely.

Bor has been the scene of intense fighting between government and rebel troops over the last week.

Nearly 400 Americans and another 300 other foreign workers have been evacuated in recent days.

Others have left on their own with commercial flights.

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"The United States and the United Nations, which has the lead for securing Bor airport in South Sudan, took steps to ensure fighting factions were aware these flights were a humanitarian mission," State department spokesperson Jen Psaki said in a statement.

"The U.S. government is doing everything possible to ensure the safety and security of United States citizens in South Sudan."

President Barack Obama said from vacation in Hawaii that 46 service members took part in the evacuation and that the US was considering its next moves in the embattled country.

Hundreds of people have been killed in South Sudan after a week of fighting, which has spread from Juba to the country's northern oil fields.

Also on Sunday, rebels reportedly seized the capital of the strategic oil-producing region, Bentiu.

There are growing concerns of an ethnic civil war in the world's newest country, said Reuters.