The Central African Republic army chief has been sacked after forces loyal to the deposed president launched a new offensive.
Some 100 people have been killed in the fighting north-west of the capital, according to officials.
Former rebel leader Michel Djotodia was sworn in as president earlier this month after his forces ousted Francois Bozize in March.
The UN has warned that CAR could become a failed state, threatening the region.
About a third of the country's 4.6 million people need assistance with food, shelter, healthcare or water, aid workers say.
Forces loyal to Mr Bozize are said to have taken at least one town since they began their attacks last week.
This is the first large-scale operation the former president's forces have staged since he was forced from power.
Medical charity Medecins Sans Frontieres (MSF) warns that the latest fighting has worsened an already precarious humanitarian situation.
It says atrocities have been committed by both sides and that many houses were burned down in Bouca, which pro-Bozize forces have reportedly seized.
"We are highly concerned about a further escalation in fighting and retaliatory acts of violence," said MSF CAR co-ordinator Sylvain Groulx.
Jean-Pierre Dolle-Waya would be replaced as army chief by Gen Ferdinand Bombayake, according to a statement read out on state radio.
Gen Bombayake was head of security for ex-President Ange Felix Patasse, who was toppled in a 2003 coup by Mr Bozize, reports the Reuters news agency.
"The change at the head of the army today is linked to the current situation in the country's north and the president's desire to bring some corrections in the security sphere," said Mr Djotodia's spokesman.
Aid workers have accused undisciplined former rebel fighters of looting the healthcare system, as well as robbing civilians, since the Seleka coalition of armed groups took power in March.
CAR has huge deposits of minerals such as gold and diamond deposits but has been plagued by chronic instability since independence in 1960.
Mr Djotodia has promised to relinquish power after elections scheduled for 2016.
Mr Bozize is currently in France after initially fleeing to Cameroon when Seleka fighters seized Bangui.