Chad has sent troops to the Central African Republic (CAR) to fight rebels who have seized Bria, a key mining town in a diamond-rich region.
CAR President Francois Bozize reportedly asked the neighbouring country for help after his own army failed to repeal the attack.
The rebel coalition accuses Mr Bozize of failing to comply with the terms of a peace treaty signed in 2007.
The CAR has had a series of rebellions and coups since independence in 1960.
In the process, Chad has intervened several times in CAR, the BBC's West Africa correspondent Thomas Fessy reports.
The nation helped Mr Bozize when he took power nine years ago and again in 2010, when he was fighting some of the rebel groups who are now on the offensive again.
The Seleka rebel coalition - formed by breakaway factions of three former armed groups - accuses the president of not honouring a ceasefire deal pledging the release of political prisoners and payment for fighters who lay down their arms.
The alliance has threatened to overthrow the government if Mr Bozize fails to enter discussions.
About 20 vehicles of heavily armed Chad soldiers crossed into CAR on Tuesday to help stop the rebel advance taking place only 300km (185 miles) from the capital, Bangui, our correspondent says.
The alliance now controls the towns of Ouadda, Sam Ouandja and Ndele, a major route linking the CAR to Sudan, Cameroon and Chad.
On Tuesday morning, it also captured the diamond mining town and military base of Bria.
Government troops tried to repel the attack launched at dawn, but were later forced to retreat. Around 15 soldiers are reported to have been killed during the clashes.
"We couldn't stand there doing nothing in front of this rebel advance," a senior government official told the Reuters news agency.
"The president contacted his counterpart in Chad, who immediately agreed to help us put a quick end to this adventure."
In the last two weeks, the rebels have stepped up their offensive against government military positions.
Mr Bozize has been in power since leading a coup in 2003 and winning the elections in 2005 and 2011.
The 2007 accord with his government led to rebel forces being integrated into the army.
But some of the rebels have since deserted and taken up arms again.
The CAR is rich in mineral resources, including gold and diamonds, but its population is extremely poor.