The rebel coalition signed a peace agreement with the government in January. The United Nations reports that rebel fighters have expressed their frustration with the government's commitment, however. They've since captured key cities.
U.S. State Department spokeswoman Victoria Nuland said the U.S. government was alarmed by the renewed violence.
"We are very concerned by the worsening humanitarian situation in CAR and credible, widespread reports of human rights abuses by both national security forces and Seleka fighters," she said in a statement. "Perpetrators of such abuses must be held accountable."
CAR is one of the poorest countries in the world and has a history of political violence. The latest rebel campaign aims to convince President Francois Bozize to step aside.
The U.N. High Commissioner for Refugees estimated there were as many as 80,000 people displaced by early 2012 conflict.
"The United States urgently calls on the Seleka leadership and on the CAR government to ensure that their forces respect the human rights of the Central African people," Nuland said.