At least 16 people have been killed during fighting in the capital of the Central African Republic (CAR), Bangui.
The city has reportedly been attacked by supporters of President Francois Bozize, who was ousted by rebels in March, plunging the country into chaos.
It comes hours before the UN Security Council is due to vote on French troops joining an African peacekeeping force.
France's ambassador to the UN, Gerard Araud, said the move would make a difference "within days".
Law and order broke down in CAR after Mr Bozize was ousted by Michel Djotodia, who installed himself as the first Muslim leader in the Christian-majority country.
Mr Djotodia has officially disbanded his Seleka rebel group and integrated some of them into the army but correspondents say it is often not clear who is in charge, even in the capital.
In the rest of the country, religious and ethnic attacks have led to warnings of genocide, with attacks on Christians by Seleka fighters and reprisals by Christian militiamen known as "anti-balaka".
The BBC's Andrew Harding in Bangui says the city is now quieter, with only the occasional burst of gunfire.
He says the attack looks like an attempt to change the balance of power before the French intervene but for now, Seleka forces remain in control.
General Arda Hakouma, a former rebel who now in charge of Mr Djotodia's personal guard, said anti-balaka fighters had attacked three parts of the city.
"There are many of them. Some of them are well armed with rifles and rocket launchers. Others are dressed in civilian clothes with machetes," he said, according to Reuters news agency.
"There has been gunfire all over town," said Amy Martin, head of the UN Office for the Coordination of Humanitarian Affairs in Bangui.
Sylvain Groulx, head of medical charity Medecins Sans Frontieres, said 16 deaths had been reported by just two hospitals in the city.
The firing began at around 06:00 (05:00 GMT) on Thursday.
Mr Araud told the BBC's Newsday programme that French and African forces were already arriving in CAR and would be able to respond quickly once the UN Security Council gives the expected "green light" on Thursday. He said all 15 members backed the resolution.
He said the troops would first restore law and order in Bangui, and then secure roads going to other main cities.
"What we understand is that a lot of people have gathered in the cities. They are totally terrified, in awful conditions, so it will allow the humanitarian aid to reach these people," Mr Araud said.
"I hope that in a few days, maybe a few weeks we are going to make a difference."
An estimated 10% of CAR's 4.6 million people have fled their homes, while more than a million urgently need food aid, according to the UN.
Aid workers say that since taking over, Seleka fighters have systematically looted the country's health centres and schools.
"The rebels have destroyed everything - there is no state," Mr Araud said.
CAR shares borders with several countries which are trying to rebuild after years of unrest, such as Chad, Sudan, South Sudan and the Democratic Republic of Congo.
The African Union (AU) has a 2,500-strong force in CAR, but is struggling to maintain law and order.
The force is expected to increase to 3,600 by the end of the month.
France says its extra military deployment in CAR - a former colony - will eventually amount to 1,000 troops supporting the AU force.
CAR has known little stability since independence in 1960 and despite its mineral wealth, most of its people remain stuck in poverty.
Mr Djotodia has been fighting Mr Bozize for many years. They signed a peace deal in 2007 but Mr Djotodia accused the ousted president of not respecting it and launched a new rebellion, backed by mercenaries from Chad and the Darfur region of Sudan.
The mainly Muslim north of CAR has known a succession of rebellions over the years - many people there accuse the mostly Christian governments of ignoring the region.