Former Darfur rebel leader Minni Minnawi has begun his role as special assistant to President Omar Bashir.
Minnawi does not want other rebels to get government positions
Mr Minnawi's faction of the Sudanese Liberation Army (SLA) was the only movement to sign a peace deal with the Sudanese government earlier this year.
He takes responsibility for government policy in Darfur, where a three-year conflict has killed tens of thousands.
On his first day, he rejected any suggestion that members of other rebel groups should get official positions.
"The deal legally was between two parties - the government of national unity and the Sudan Liberation Movement (SLM)," he told reporters in the Republican Palace in Khartoum
"Our argument is not just about political posts but from a legal point of view whoever will be appointed has to be someone from the SLM," he said.
Correspondents say Mr Minnawi's appointment is unpopular with some politicians, who are strongly opposed to the Darfur rebel groups.
Recently Mr Minnawi's soldiers participated in joint actions with the Sudanese army against the rebel Justice and Equality Movement (JEM), which did not sign the deal.
Some 2m people have fled their homes in Darfur since conflict began in 2003, and tens of thousands of people are reported to have been killed in ongoing violence.
SLM Minnawi was the main rebel group in the Darfur conflict, and the only one that signed the deal on 5 May with the Sudanese government.