Government forces in the Central African Republic (CAR) say they have halted an advance by insurgents towards the capital, Bangui.
Reports say an attack helicopter opened fire on the column of Seleka rebels as it approached the city.
Rebels earlier forced their way through a checkpoint 75km (45 miles) north.
The checkpoint, in Damara, was manned by a regional force which had designated it as a red line the rebels should not cross.
But the atmosphere in Bangui is very tense and all businesses, offices and schools have shut, a journalist in the capital told the BBC.
Seleka began their offensive this week, accusing President Francois Bozize of failing to honour a peace deal signed in January - a charge he denies.
The rebels joined a power-sharing government in January, in talks brokered by regional leaders, to end the rebellion they launched last year.
Journalist Junior Lingange in Bangui told the BBC's Focus on Africa programme that the rebels said 2,000 of their fighters had taken the checkpoint in Damara, where some 500 troops from the regional Fomac force were based.
Damara is about an hour's drive from the capital, he says. In Bangui, government troops were driving around the capital, where some 400 South Africa troops are also based, said the journalist.
"The rebel column, which was headed south, was stopped by an aircraft... an attack helicopter," a senior regional military source told Reuters news agency.
"The helicopter opened fire on the column, forcing it to disperse ... The rebels have not reached Bangui."
CAR has been hit by a series of rebellions since independence from France in 1960.
It is one of the poorest countries in Africa, despite its considerable mineral resources.