* Rebel advance risks unleashing new spiral of violence
* Parliament speaker: "The life of the nation is in danger"
* Instability has hit investment in mineral-rich nation (Releads with Chadian troop deployment, writes through)
By Paul-Marin Ngoupana and Madjiasra Naka
BANGUI/N'DJAMENA, Dec 18 (Reuters) - Chad sent troops into neighbouring Central African Republic on Tuesday to help the government there stop a rapid advance by rebels that risks sparking a new spiral of violence in the mineral-rich nation.
The rebels killed 15 government soldiers during the capture on Tuesday of Bria, a diamond mining town about 600 km (360 miles) northeast of the capital Bangui and the sixth town to fall to the insurgents in little over a week.
The rebels - made up of breakaway factions from groups which signed a 2007 peace accord - threatened on Monday to overthrow President Francois Bozize if he failed to honour the deal.
"There was a column that passed through Kabo heading for Kaga-Bandoro very early this morning. There were more than 20 vehicles with soldiers aboard," said a U.N. official, who asked not to be named.
A senior officer with Chad's army general staff and a CAR government minister, both also speaking on condition of anonymity, confirmed the deployment.
"We couldn't stand there doing nothing in front of this rebel advance," the minister told Reuters.
"The president yesterday contacted his counterpart in Chad, who immediately agreed to help us put a quick end to this adventure," he said, adding that Bozize had travelled to the front on Monday to personally take command of operations.
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 rebellions, banditry, ethnic tensions and the spill-over of conflicts in 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 with support from Chad's President Idriss Deby and won a new mandate in January 2011 elections which opponents dismissed as fraudulent.
The two leaders remain close allies, and Chad has intervened in CAR on several occasions in support of Bozize.
"LIFE OF NATION IN DANGER"
Bria residents told Reuters by phone they were woken by small and heavy weapons fire at around 5am (0400 GMT) on Tuesday and within hours the rebels had routed government troops stationed there.
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.
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.
The rebel alliance, known as SELEKA, demanded on Monday that 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.
A day earlier on Sunday, the rebels ambushed government soldiers sent to retake the town of 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.
Since last week the rebels have taken a string of towns in the north and northeast including Ndele, Mbres, Sam Ouandja, Bamingui and Ouadda, according to army sources and witnesses.
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.
(Additional reporting by John Irish in Paris and Bate Felix in Dakar; Writing by Bate Felix and Joe Bavier; Editing by Stephen Powell)