The US military is to fly African forces into Central African Republic, responding to a request by France to bolster international efforts to halt the spread of violence between Christians and Muslims.

Two US military C-17 aircraft will fly about 850 troops from Burundi into Central African Republic within the next 24 hours, Lieutenant Colonel Robert Firman said.

It is unclear what US support might follow, but Lt Col Firman said consultations were ongoing.             

Pentagon spokesman Carl Woog said the military was working to identify additional resources that could help address further requests for assistance.

"The United States is joining the international community in this effort because of our belief that immediate action is required to avert a humanitarian and human rights catastrophe," Mr Woog said in a statement.             

More than 400,000 people have been displaced since Seleka rebels - many of them Muslims from neighbouring Chad and Sudan - seized power in March, unleashing a wave of rapes, massacres and looting on the majority Christian population.

Over 400 people have died since Thursday in the capital Bangui alone.

The UN Security Council on Thursday mandated France to do whatever necessary to protect Central African Republic's 4.6m people and restore government authority while an African Union peacekeeping mission slowly deploys.

The forces from Burundi will help bolster the contingent from the African Union, due to be increased to 6,000 from about 3,500.

France has moved 1,600 troops to its former colony.

French troops earlier exchanged fire with former rebels in Bangui.

The French troops were searching for weapons. Shooting erupted near the airport after gunmen refused to hand over their weapons.

French forces later came under attack by former rebels in the city centre.

France said it was prepared to use force if fighters rejected calls to disarm or return to barracks.             

US Defense Secretary Chuck Hagel authorised the US intervention after speaking to his French counterpart, Defence Minister Jean-Yves Le Drian, and receiving a request for limited assistance, Mr Woog said.

"In the near term, France has requested airlift support to enable African forces to deploy promptly to prevent the further spread of sectarian violence in the Central African Republic," Mr Woog said, without ruling out further support.

The US military extended similar airlift assistance for French forces heading to Mali earlier this year in a campaign against al-Qaeda-linked extremists.

France's intervention in Central African Republic may prove simpler militarily than the one in Mali, where well-trained rebels were willing to die for their cause and had months to create arms caches and mountain hideouts before the French arrived.

Seleka, by contrast, is an ill-disciplined coalition of warlords whose poorly-armed foreign fighters came for plunder and may disperse rather than fight, defence officials say.