N'DJAMENA (Reuters) - Chad sent troops to neighboring Central African Republic on Tuesday to help government troops there stop a rapid advance by rebels that risks sparking a new spiral of violence in the mineral-rich nation, Chadian and United Nations officials said.

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 onto a 2007 peace agreement - threatened to overthrow President Francois Bozize on Monday if he failed to honor the five-year-old 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, also speaking on condition of anonymity, confirmed the deployment.

"It was a request from (CAR) President (Francois) Bozize," he told Reuters.

(Reporting by Madjiasra Nako; Additional reporting by Joe Bavier in Abidjan; Editing by Stephen Powell)