The embassy shutdown, which the State Department described as temporary, was "a result of the deteriorating security situation" in the landlocked Central African country of 4.4 million, the department said in a statement.
The department said the shutdown did not mean Washington had suspended diplomatic relations with the CAR, bordered by Chad, Sudan, South Sudan, the Democratic Republic of the Congo, the Republic of the Congo and Cameroon.
"The Department of State warns U.S. citizens against all travel to the Central African Republic," the department said. "U.S. citizens who have decided to stay in CAR should review their personal security situation and seriously consider departing, taking advantage of commercial flights."
U.S. officials told CBS News U.S. Ambassador Laurence D. Wohlers and some 40 others flew out of the capital, Bangui, on a U.S. Air Force C-40 Clipper for Kenya.
A C-40 is a military version of the Boeing 737.
Rebel forces calling themselves the Seleka Coalition captured many towns in the impoverished country's central and eastern regions in the past 18 days and were now advancing toward Bangui and were less than 180 miles away, several news organizations reported.
Residents in the capital of 600,000 people feared the insurgents -- trying to remove President Francois Bozize, a military officer who seized power in 2003 -- could attack very soon, CBS News reported.
The rebels accuse Bozize, who has twice been elected president after seizing power, of failing to live up to the terms of peace agreements signed since 2007.
Bozize appealed to the international community for help Thursday, especially France, which ruled the country until 1960.
His appeal came as protesters threw stones and tore down the French flag at the French Embassy, demanding the former colonial power do more to stop the rebels.
French President Francois Hollande rejected the appeal, saying France's 250 soldiers near Bangui were part of a multinational peacekeeping force and would protect the more than 1,200 French nationals in the country but would not defend Bozize's regime.
The era of French intervention in the internal affairs of its former colonies "is over," Hollande told reporters.
A French Foreign Ministry spokesman nonetheless condemned "the continued hostility by the rebel groups" and said the crisis should be resolved through dialogue.
The United Nations also pulled non-essential staff out of the country, the world body said.
The U.N. Security Council issued a statement late Thursday reiterating its concern about the crisis and condemning the attacks.
Washington, to which Bozize also appealed for help, expressed "deep concern" about the rebel attacks but had no immediate public response to the appeal for help.