France rushed troops to Central African Republic on Friday, its second major African intervention in a year, to stem Muslim-Christian violence that has claimed more than 185 lives this week and threatens to broaden into civilian massacres.

Hundreds of soldiers started arriving from neighbouring countries, though Paris said a small detachment of French forces already stationed at the airport in the capital Bangui killed a number of unidentified fighters nearby on Thursday, hours before being given a UN green light for the mission to restore order.

Joanna Mariner, part of an Amnesty International team in Bangui, said that she had reports of pillaging and killing in the 3rd district.

"The French are patrolling on the main axes, but the city isn't yet secure," she said.

Bangui residents were cautiously optimistic over French deployments but a witness in the neighbourhood of PK12 said mainly Muslim ex-rebels had killed several people in raids on houses.

Rebels seized power in March

The Archbishop of Bangui said 39 people were killed overnight and on Friday. Meanwhile, clashes also continued in Bossangoa, to the north, where at least 30 people had been killed, according to an aid worker.

The former French colony has slipped into chaos since mainly Muslim rebels seized power in March, leading to tit-for-tat violence with the Christian majority. The violence on Thursday was the worst the capital has seen during the crisis.

France, which halted an advance by al Qaeda-linked insurgents on the Malian capital Bamako this year, began assembling a new 1,200-strong force for CAR just hours after winning UN backing.

The French operation was codenamed Sangaris, after a red butterfly found in the country.

"The operation has effectively started," Defence Minister Jean-Yves Le Drian told RFI radio.

He said one company had arrived in Bangui from a French base in nearby Gabon and that a helicopter group was due to be in place later in the day.

He described the night as calm after fighting on Thursday between the mainly Muslim former rebels now in charge of the country and a mix of local Christian militiamen and other fighters loyal to ousted president Francois Bozize.

PM wants French to stay for a year

A Reuters witness and an aid worker said at least 105 people were killed in Bangui on Thursday. At least 12 civilians died in an attack outside the capital earlier in the week.

Central African Republic President Michel Djotodia is the former leader of the Seleka rebel alliance that overthrew his predecessor. (Joe Penney/Reuters)

Le Drian said it was "not impossible" that France could wind down its presence after six months, but Central African Republic Prime Minister Nicolas Tiangaye said it was likely the troops would have to remain longer.

"Six months seems a bit short to me; in my view, we are looking at a year," Tiangaye told RTL radio. "If it (the French force) manages to sort out the problems, so much the better, but I would prefer it to stay in place for a year."

In a sign of broadening involvement, Britain said it would provide aircraft to transport French equipment.

Central African Republic is rich in gold, diamonds and uranium, but decades of instability and spillover from conflicts in its larger neighbours have kept it mired in crisis.

African forces helping secure other towns

The streets of Bangui were largely quiet on Friday, with just a handful of vehicles carrying French troops and former rebels, known as Seleka, though rain-sodden streets.

A French war plane passed overhead.

They are looting, and they are killing people. - Bangui resident

A resident in the PK12 neighbourhood who did not want to be named for his own safety said Seleka fighters were "going door-to-door."

"They are looting, and they are killing people," he said. "They are calling everyone 'anti-balaka'."

Anti-balaka is the term used to describe the Christian militias, vigilante groups and so-called "self-defence" units that are fighting the Seleka rebels.

Giving the latest toll for Bangui, Archbishop Dieudonne Nzapalainga warned, "This is a cycle of violence."

Fabius said French forces would initially focus on securing Bangui and roads leading to Chad and Cameroon. They would also deploy with African forces to other towns, including Bossangoa, about 300 km north of the capital, which witnesses said had come under heavy fire from former rebels on Thursday.

Some fear massacre to come

Highlighting the extent of the challenge facing French forces, an aid worker in Bossangoa, where tens of thousands of people, mainly Christians have fled their homes, said fighting between communities continued there on Friday. Around 30 dead had been counted from Thursday's violence.

Dieudonne Yanfeibona, a priest at the mainly Catholic mission said: "Seleka are now burning down the neighbourhood all around. There's a risk that they will commit a massacre."

An African peacekeeper died of wounds sustained protecting civilians in Bossangoa, the African mission said.

Michel Djotodia, leader of the Seleka ex-rebel alliance, is CAR's interim president, but he has struggled to control his loose band of fighters, many from neighbouring Chad and Sudan.

Asked whether Djotodia was legitimate and should remain in power, Fabius said he had taken power "in a debatable way" but added: "I think we don't need more difficulties by adding the departure of the president."

He said, however, that elections should begin by early 2015 at the latest.