President Barack Obama said 50 US troops have deployed to the African country of Chad to help evacuate US citizens and embassy personnel from the neighbouring Central African Republic's capital of Bangui in the face of rebel advances toward the city.
Mr Obama told congressional leaders of Thursday's deployment in a letter citing a "deteriorating security situation" in the Central African Republic.
The evacuation of the US diplomats comes after criticism of the Obama administration's handling of diplomatic security at its consulate in Benghazi.
The ambassador and three other Americans were killed in a September 11 attack.
In the Central African Republic, rebels have seized at least 10 northern towns. On Saturday they continued their advance, seizing the city of Sibut, 114 miles from Bangui.