Thierry Vircoulon, Central Africa project director for the think tank International Crisis Group, told IRIN that the rebels were "progressing quite fast and they constitute a real threat for the regime".
"They managed to unite and they are sufficiently well equipped to challenge the CAR's army and, except for the Chadian army, no force can prevent them from taking the road to Bangui at this stage," he said.
An earlier alliance statement set out a long list of political and military demands and made a cessation of hostilities conditional on the government's agreement to hold talks with them.
The rebels want: the implementation of the recommendations of the Inclusive Political Dialogue, which was held in 2008 among government, civil society, the opposition and the rebels; financial compensation for the rebels; the release of political prisoners; and the opening of an investigation into the disappearance of former CPJP (Convention of Patriots for Justice and Peace) leader Charles Massi and other "crimes".
The statement denounced, among other things, "the exclusion and the contempt, the favouritism and the tribalism" of the head of state and his family, as well as "the looting of the wealth of Central Africans by the government authorities who are supposed to protect them."
The authorities have not responded officially to the seizure of Bamingui or Bria, or to the creation of the new alliance, though the latest army communiqué, dated 12 December, noted the rebel capture of Ndélé.
"The capture of these towns is a direct challenge to the government," Vircoulon said. "The demands of the rebels are very clear and illustrate a high level of dissatisfaction with the peace process. They basically consider that the peace process is unfinished business and needs to be reactivated."
The new politico-military alliance is called Seleka CPSK-CPJP-UFDR, and was officially launched in a press release signed by the three leaders on 16 December. It is made up of the Wa Kodro Salute Patriotic Convention (CPSK), chaired by Nureldine Adam; CPJP, chaired by Dhaffane Mohamed Moussa; and a dissident faction of the Union of Democratic Forces for Unity (UFDR), chaired Michel Djotodja.
The first two groups have signed peace deals with the government and were supposed to have laid down their arms.
A number of government soldiers have been captured or are missing. Twenty-two soldiers captured during the seizure of Sam-Ouandja on 10 December are still in the hands of their captors. Additionally, between 10 and 30 been untraceable since the fall of Bamingui, according to sources that also mention the disappearance of vehicles and other military equipment.
This sudden resurgence of hostilities in northern CAR could jeopardize the peace process and the signing of the Comprehensive Peace Agreement between the government and three rebel movements - APRD (Popular Army for the Restoration of Democracy); UFR (Union of Republican Forces); and UFDR.
[ This report does not necessarily reflect the views of the United Nations. ]