State Department spokeswoman Victoria Nuland said the U.S. government had serious concerns about the stability in CAR. Her comments followed a decision by the rebel Seleka coalition to appoint Michel Djotodia as president.
"The only legitimate government in the CAR is the government of national unity led by Prime Minister Nicolas Tiangaye," she said in a statement.
The International Committee of the Red Cross expressed concern about the humanitarian situation in CAR last week. Hospitals are running out of medicine and the capital is low on electricity and water supplies.
The Seleka alliance reached a truce with the CAR government in January but resumed the fight after saying the peace measure broke down. CAR President Francois Bozize has fled to Cameroon.
The U.S. government said Seleka's decision represented an illegal takeover but blamed both sides for the political collapse.
"We urge the country's leadership to quickly establish an open and legitimate process leading to presidential elections and the reestablishment of a constitutional government," Nuland said.