The United Nations is evacuating non-essential staff from the Central African Republic as rebels advance towards the capital, Bangui.
The United States has also urged its nationals to leave, the BBC reports.
There is a mood of deep anxiety in the capital of the Central African Republic, Bangui, as rebels advance towards the city, a UN envoy has said.
Margaret Vogt told the BBC she had seen that residents of Bangui were "petrified as to what could happen".
The UN is now evacuating its non-essential staff from the country, while the US urged its nationals to leave.
France has ordered tighter security of its embassy in Bangui after it was attacked by protesters.
The demonstrators threw stones and tore down the French flag.
They want France to help quash a rebellion in the north of the country, accusing the former colonial power of abandoning them.
France has about 200 soldiers based there, and the government in Bangui has appealed for France to intervene against a rebel movement which has taken several northern towns.
Analysts say Paris is increasingly reluctant to interfere in its former colonies and a spokesman for the French Foreign Ministry told Reuters news agency the crisis needed to be resolved through dialogue.
Rebels in northern CAR on Sunday captured the key city of Bambari, the third largest in the country, having earlier seized the rich diamond mining area around Bria.
The rebels - known as the Seleka coalition - accuse President Francois Bozize of failing to honour a 2007 peace deal, under which fighters who laid down their arms were meant to be paid.
The rebels say they have no intention to march to the capital, but contradictory messages have pushed the UN and the US to take preventive measures, the BBC's West Africa correspondent Thomas Fessy reports.
Seleka, which is made up of breakaway factions from three of the former armed groups, has pledged to depose Mr Bozize unless he negotiates with them.
They began their campaign a month ago and have taken several towns in their push towards the capital.
It is unclear how far they have advanced towards Bangui, but an unconfirmed report by Reuters news agency on Sunday quoted sources saying they were only 75km away.
President Bozize, who seized power in a coup in 2003, has repeatedly relied on foreign intervention to fend off rebellions and the spill-over from conflicts in neighbouring Chad and Sudan.
Chad has deployed 150 soldiers to try to stem the rebel advance.
Air France's weekly flight from Paris to Bangui was turned back in mid-air.