The cease-fire, signed Jan. 11 to end a rebel insurgency that began in the impoverished country last year, allowed for President Francois Bozize to remain in power, but required him to dissolve the government and name a new prime minister.
The new prime minister would create a temporary coalition government that would introduce social, economic and judicial reforms, bring rebel soldiers into a new national military and set up new legislative elections.
Bozize appointed Nicolas Tiangaye prime minister Jan. 17.
But Bozize didn't quickly implement another part of the cease-fire requiring him to free all political prisoners taken during the conflict. The accord also called for all foreign troops to return to their countries of origin.
The rebel Seleka coalition was not required under the agreement, signed in Libreville, Gabon, to give up the cities it seized until Bozize implemented his part of the accord.
Seleka recently renewed attacks this month and took over a key southeastern town.
"We call on President Bozize and the leadership of the Seleka alliance to cease hostilities immediately, and implement the provisions of the Libreville Agreement," State Department spokeswoman Victoria Nuland said in a statement.
"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. Perpetrators of such abuses must be held accountable," Nuland said.
A U.N. envoy cited reports last week of widespread rape, looting, recruitment of children as fighters and a starvation risk as villagers, hiding in the bush in fear, were unable to tend to their crops.
"There is a feeling of helplessness," Margaret Vogt, Secretary-General Ban Ki-moon's special representative, told reporters in New York.
"Without a strong response from the international community there is no future," she said.
The State Department told all U.S. citizens Dec. 24 to avoid all but essential travel outside the capital Bangui. It also evacuated all non-essential personnel from the U.S. Embassy.
Four days later the department suspended embassy operations due to the ongoing rebel attacks. Ambassador Laurence D. Wohlers and his diplomatic staff evacuated the country.