* CAR rebels attack mining town on Tuesday
* Rebels demand government implement 2007 peace deal
* Instability has hit investment in resource-rich nation (Adds French reaction, seized towns)
By Paul-Marin Ngoupana
BANGUI, Dec 18 (Reuters) - An alliance of rebel groups that has seized control of several towns in Central African Republic in recent days has threatened to overthrow President Francois Bozize if he fails to fully implement a five-year-old peace deal.
The resurgence of rebel activity risks sparking a new spiral of violence in the minerals-rich nation where several groups have fought low-level insurgencies since 2005.
The rebel alliance - made up of breakaway factions from the CPJP, UFDR and CPSK rebel groups in the north of the country which signed a peace treaty with the government in 2007 - said the government must fully implement the terms of that deal.
In a statement issued on Monday, the alliance demanded, among other things, that the government pay rebel soldiers money promised to them to lay down their weapons, and to free political prisoners.
"Otherwise ... (the alliance) will take it upon itself to do everything possible to change, sooner or later, this regime which has done nothing to bring justice and peace to the Central African Republic. Enough is enough," it said.
Early on Tuesday, the rebels entered Bria, a mining town with a population of about 40,000 which lies some 600 km (360 miles) from the capital Bangui.
"Fighting started about 5 a.m. today. We were woken by heavy gunfire. Most of the population started fleeing. The military base seems to be in the hands of the rebels," Bria's Mayor Moussa Gouman told Reuters by telephone as gunfire could be heard in the background.
A resident said that by late afternoon the rebels had seized control of the town.
The rebels have taken several towns in the north and north-east of the country including Ndele, Sam Ouandja, Bamingui and Ouadda during an advance that began last week.
Authorities in the capital declined to comment.
On Sunday, the rebels attacked government soldiers sent to retake Ndele which was seized on Dec. 10.
"Our troops were ambushed by the rebels on (Sunday) night. Forty of them are unaccounted for as we speak," a CAR army captain told Reuters, requesting not to be named.
"Two of our vehicles loaded with weapons, ammunition and fuel supplies for our men were also seized by the rebels," he said.
Instability in landlocked CAR, roughly the size of former colonial master France, has discouraged major investment in its timber, gold, uranium and diamond deposits.
France said it was "extremely concerned" by the fighting.
"These attacks constitute a flagrant violation of the Libreville peace agreement, in that they undermine efforts to consolidate peace in Central African Republic," French Foreign Ministry spokesman Philippe Lalliot told journalists.
A 2006 advance by the nearly 3,000-strong UFDR on Bangui was only halted with the intervention of French armed forces before the peace deal was signed in April 2007.
The CPJP embarked on a separate rebellion with about 1,000 soldiers before agreeing a ceasefire, but most of the rebel groups remained armed.
A mix of local rebels, bandits, ethnic tensions and the spillover of conflicts from neighbouring Chad, Sudan and Democratic Republic of Congo have undermined efforts to stabilise the nation which has suffered misrule since independence in 1960.
President Bozize took power in a 2003 coup and won a new mandate in January 2011 elections, which opponents dismissed as fraudulent. (Additional reporting by John Irish in Paris and Bate Felix in Dakar; Writing by Bate Felix; Editing by Robin Pomeroy)