"These developments gravely undermine the peace agreements in place and the efforts of the international community to consolidate peace in the Central African Republic," a representative for Ban said in a statement issued Wednesday from New York.
Earlier this week, the Seleka rebel coalition seized a regional capital, Kaga-Bandoro, and was reportedly within 200 miles of Bangui, the national capital.
The republic, a landlocked country bordered by Chad, Cameroon and the Democratic Republic of Congo, is among the poorest countries in the world.
Ban appealed to all parties to refrain from acts of violence and "to ensure the protection of civilians and to respect human rights, in conformity with international instruments to which the Central African Republic is a party."
The State Department issued a travel warning Monday, recommending against "all but essential travel outside the capital, Bangui."
An emergency message for U.S. citizens issued Tuesday said the embassy was "suspending normal operations until further notice." The State Department encouraged U.S. citizens to leave Central African Republic "until the security situation improves."
Pro-government protesters in Bangui threw rocks at the French Embassy Wednesday after calls for aid and military intervention to repel the rebels were rebuffed by French President Francois Hollande, the British newspaper The Telegraph reported Thursday.
Central African Republic was part of a colony known as French Equatorial Africa until independence from Paris in 1960.
"If we are present it is not to protect a regime, it is to protect our nationals and our interests, and in no way to intervene in the internal affairs of a country, in this case Central Africa. Those days are over," Hollande said.