South Africa has authorised the sending of 400 soldiers to Central African Republic (CAR) where government and regional troops are battling a rebel insurgency.

"President Jacob Zuma has authorised the employment of 400 South African National Defence Force personnel to the Central African Republic, to render support in fulfilment of an international obligation of the Republic of South Africa towards the CAR," Zuma's office said in a statement.

Their mandate is through March 2018, it added.

The South African presidency said the soldiers would help with capacity-building of the Central African Republic defence force.

It "will also assist CAR with the planning and implementation of the disarmament, demobilisation and re-integration processes", it added.

Earlier a military source said at least 200 South African soldiers had arrived in the capital Bangui to try to secure it from advancing rebels.

"This well-equipped South African contingent arrived in the middle of the week. ... Its mission is to secure the Central African capital," the source told AFP.

The troops, based not far from the neighbourhood housing the residence of President Francois Bozize, "joins in Central Africa a South African military contingent already deployed as part of military cooperation", the source added.

After the South African troops arrived, the coordinator of the Citizens Coalition Opposed to the Armed Rebels, Levy Yakite, appealed Sunday on national radio for his movement to remove roadblocks set up to prevent attempts by the rebels to infiltrate.

With the Seleka rebel coalition threatening to march on Bangui, Gabon, Congo and Cameroon in recent days have each sent 120 troops, according to a source with the multinational African peacekeeping force FOMAC.

They will join 400 Chadian soldiers already deployed to protect Damara, the last town on the rebels' road to Bangui, the source said.