Secretary of State John Kerry, in a statement issued by the State Department, called on CAR's interim government to "immediately end" the violence.
Fighting between Seleka and Anti-Balaka fighters in the capital Bangui Tuesday and Wednesday left dozens of people dead, including civilians and African Union troops, and caused residents of the northern part of the city to flee their homes, the statement said.
"We were deeply disturbed by the discovery on Dec. 26 in Bangui of a mass grave containing over 20 bodies," the statement said. "The continued sectarian fighting only deepens the country's wounds and makes reconciliation more difficult."
Kerry called for CAR officials to hold "fair and inclusive" elections by February 2015.
He commended the African-led mission to stabilize the country, AU troops and French forces "for their commitment to ending the violence and establishing an environment in which a political transition to a democratically elected government can take place."
The statement called on CAR transitional authorities "to take every possible step to end violence and promote reconciliation" and to support efforts by French and AU troops to disarm Seleka and Anti-Balaka forces.