Catherine Samba-Panza, the interim president of the Central African Republic, has chosen Mahamat Kamoun as the country's first Muslim prime minister.
The appointment of Kamoun, 53, was announced on state radio on Sunday, days after his predecessor stepped down as part of a government reshuffle.
Kamoun is tasked with forming a consensus government and guiding the country to elections next year in a bid to end sectarian violence that has killed thousands of people and displaced around one million.
Samba-Panza took office in January when Seleka leader Michel Djotodia resigned amid an international outcry over abuses by the mostly Muslim rebels after they seized power in March 2013 in the majority Christian country.
The Christian anti-balaka militia then took up arms against them.
Kamoun, an economist, was the cabinet chief for Djotodia during his presidency, but the rebels say he is not a member of Seleka.
Kamoun has served as an adviser to Samba-Panza since Djotodia's resignation, the Reuters news agency reported.
Seleka rebels said on Monday they would not take part in a national unity government as they were not consulted about the choice of prime minister and said they may rethink last month's ceasefire deal agreed last month in the capital of neighbouring Congo Republic.
"The transitional head of state did not think to open talks with Seleka and just decided to name a prime minister," General Mohamed Mousa Dhaffane, the group's second vice-president, said in a statement.
When Seleka withdrew from the southern capital Bangui after Djotodia's resignation, tens of thousands of Muslims fled militia reprisals to a northern rebel enclave.
Persistent fighting on the edge of rebel territory has undermined the ceasefire with the anti-balaka militia.
An anti-balaka spokesman said they were happy with Kamoun's nomination and would take part in a unity government.
Speaking on state radio, Kamoun said he would name a cabinet of about 26 members.
It would focus on re-establishing security, improving the humanitarian situation, fostering national reconciliation, relaunching government administration and the economy, and organising elections, he said.
The UN is due to deploy a 12,000-strong peacekeeping mission next month, much of it made up of a 6,000-strong African mission already on the ground.
Last December, France deployed 2,000 peacekeepers to stem the sectarian violence.