The rebels, who already have control of four other regional capitals in the centre and north of the country, faced no resistance as they entered the town of Sibut around 150km from Bangui, a military official told Agence France-Presse.
The streets of Bangui were deserted on Saturday night, according to an AFP journalist, after a curfew was imposed from 7pm to 5am (6pm GMT to 4am GMT).
Many shops were being guarded by men armed with machetes. "The bosses fear looting so they are paying guards," said one guard.
Officials on both sides said the rebels of the so-called Seleka coalition had also repelled army soldiers trying to recapture Bambari, a former military stronghold in the landlocked country, one of the world's poorest despite vast mineral wealth.
A military official described "extremely violent" fighting over the town, with detonations and heavy weapons fire audible to witnesses some 60km away.
The rebel advance on Sibut, also a base for Chadian soldiers stationed in the country, forced government forces and their allies to retreat to Damara, 75km from Bangui and the last major town on the road to the south-western capital.
"The rebels entered Sibut. There was no fighting, the Central African Armed Forces (FACA) stationed there and the Chadian troops left the town last night [Friday] for Damara," the military official told AFP.
Djouma Narkoya, a Seleka leader, claimed that the army suffered "losses" in the fighting for Bambari, while the rebel side had "one killed and three injured" in the fighting.
"We are continuing to progress," he added.
Sibut residents arriving in Bangui said they saw around 60 Chadian and Central African army vehicles converging on Damara late on Friday.
One of the towns under the control of the rebels, who launched their offensive in early December, is the garrison town and key diamond mining hub of Biraosince.
Former colonial power France, meanwhile, boosted its military presence to 400 on Friday with the deployment of 150 paratroopers to Bangui airport, and the Economic Community of Central African States (ECCAS) announced reinforcements.
French Prime Minister Jean-Marc Ayrault stressed again on Friday that French troops were there only to protect French and European nationals, not fight the rebels.
Regional efforts to mediate a peaceful solution in the landlocked equatorial country were at a standstill.
A day after announcing that the rebels and the government had agreed to hold unconditional peace talks and that more regional troops would head to the country, ECCAS said no dates had been set for either move.
The bloc's foreign ministers will meet again next Thursday "and that is when they will announce a date for the meeting in [the Gabonese capital] Libreville," ECCAS's communications director Placide Ibouanga told AFP, referring to talks between rebels and the government.
The coalition of three rebel movements known as Seleka — or the "alliance" in the Sango language — says the government has not fulfilled the terms of peace pacts signed in 2007 and 2001, providing for disarmament and social reintegration for insurgents, including pay.
Central African President Francois Bozize, who took power in a 2003 coup, has twice been elected into office.
Bozize's appeals for help from France and from the United States to fight the rebels have fallen on deaf ears.
Neighbouring Chad, which has helped Bozize with rebellions in 2010, earlier sent a contingent to the country, however.
In Bangui, food prices have soared, further spiking tensions and uncertainty.
"I'm afraid of the rebels coming," said vegetable vendor Euphrasie Ngotanga in the city's huge Sambo market. "We're not going to sell our produce if there's no peace. And then how we will feed our children?"
"We don't eat properly any more," said another vendor, Angele Bodero, with her baskets full of condiments before her. "Cassava has become more expensive, everything costs more," she said, referring to the country's staple food.
A bag of cassava has risen nearly 50% from 13 000 CFA francs to 18 000 FCFA ($26 to $32). – AFP