* Rebel advance risks unleashing new spiral of violence
* Lawmakers hold pro-army rally outside parliament
* Parliament speaker: "The life of the nation is in danger"
* Instability has hit investment in mineral-rich nation (Recasts with attack on Bria)
By Paul-Marin Ngoupana
BANGUI, Dec 18 (Reuters) - Rebel groups killed at least 15 soldiers as they seized a diamond mining town in Central African Republic on Tuesday, witnesses said, the latest sign of an offensive that risks provoking a new spiral of violence in the mineral-rich nation.
The attack on Bria, about 600 km (360 miles) northeast of the capital Bangui, came a day after the alliance of insurgent movements threatened to overthrow President Francois Bozize if he failed to honour a five-year-old peace deal.
Lawmakers walked out of parliament in Bangui on Tuesday as news of the attack spread and held a rally in support of the army.
"The moment is delicate. The life of the nation is in danger," parliament speaker Celestin Leroy Gaombalet said.
Long-running instability in landlocked CAR, roughly the size of former colonial master France, has discouraged major investment in its timber, gold, uranium and diamond deposits.
A mix of local rebels, bandits, ethnic tensions and the spill-over 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.
The new rebel alliance is made up of breakaway factions from the CPJP, UFDR and CPSK - insurgent groups in the north of the country who signed a peace treaty with the government in 2007.
It has taken a string of towns in the north and northeast including Ndele, Sam Ouandja, Bamingui and Ouadda in an advance that began last week, according to army sources and witnesses.
Bria residents told Reuters by phone they were woken by small and heavy weapons fire at around 5am (0400 GMT).
"The rebels took control of the town around midday," resident Francois Leki told Reuters by telephone.
A military source, who asked not to be named, confirmed the army had lost the town to the rebels.
"Yes, I can unfortunately confirm the fall of Bria and the deaths of around 15 of our men ... There are also many missing among our ranks," he said.
In a statement issued on Monday, The rebel alliance demanded the government free prisoners and pay rebel soldiers money promised to them to lay down their weapons, among other demands.
"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.
The rebels ambushed government soldiers sent to retake Ndele which was seized on Dec. 10, an army officer told Reuters.
Forty soldiers remain unaccounted for following the attack, in which the rebels took two vehicles loaded with weapons, ammunition and fuel, the officer said, asking not to be named.
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.
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 and Joe Bavier; Editing by Andrew Heavens)