UN experts accuse Abdoulaye Miskine, who signed a peace deal with the government last year, of looking for fighters.
The Central African Republic says Russia and Rwanda sent hundreds of troops into the country after an alleged coup bid that took place ahead of the presidential and parliamentary polls scheduled for next week.
On Saturday, the government of the CAR accused former president Francois Bozizé of an attempted coup after three powerful rebel groups merged and started to advance on the capital Bangui.
“Russia has sent several hundred soldiers and heavy weapons” in the framework of a bilateral cooperation agreement, government spokesman Ange Maxime Kazagui said on Monday.
“The Rwandans have also sent several hundred men who are on the ground and have started fighting.”
Rwanda confirmed the deployment, saying the move was in response to the targeting of its troops in the UN peacekeeping force by rebels supported by Bozizé, who ruled the CAR from 2003 to 2013.
There was no immediate confirmation from Moscow although the Kremlin voiced “serious concern” about events in CAR.
Private security guards employed by Russian companies already provide protection for President Faustin-Archange Touadéra and are involved in the training of local armed forces.
The opposition has called for the December 27 vote to be cancelled until “peace and security” are restored.
The CAR spiralled into a conflict in 2013 when Bozizé was overthrown as the president by the Seleka, a rebel coalition drawn largely from the Muslim minority.
The coup triggered a bloodbath between the Seleka and so-called “Anti-balaka self-defence forces”.