The UN has given the green light to a beefed-up military operation by France and African troops in the Central African Republican (CAR) after more than 100 people died in fighting in the capital of Bangui.
Christian fighters backing deposed president Francois Bozize attacked the city before dawn and clashed with Muslim militia supporting the government of President Michel Djotodia.
The UN Security Council authorised the African Union (AU) to add 1,000 troops to its 2,500-strong force in CAR and to use "appropriate measures" to protect civilians and restore security.
The French-sponsored resolution also allowed Paris to back the AU mission for a temporary period. Paris has pledged an additional 1,000 troops.
"People have been terrorised by militia carrying out atrocities," said France's ambassador to the UN, Gerard Araud.
"On the security front, the country risks collapsing into chaos, with uncontrollable and unforeseen consequences for the whole region," he said. "On the human front, the risk of mass atrocities is there.
"History demands we avoid the worst," he said
Bozize was ousted by Djotodia through a military coup in March. The latter headed the Seleka rebel coalition, which was made up of mostly Muslims, and appointed himself as the first Muslim president of CAR. Half of the country's 4.5m population is Christian, while about 15% is Muslim.
The move sparked the creation of the so-called "anti-balaka" (machete in the local Sango language) Christian self-defence force.
Both factions have been accused of atrocities against the population.
US ambassador Samantha Power commended France and AU's joint effort.
"What matters right now to the civilians whose lives are hanging in the balance is not the colour of the helmet of those tasked to protect them," she said. "What matters is whether the troops there move out aggressively to protect civilians and to restore security."
In Whitehall, the Ministry of Defence said it was considering providing France with "limited logistical support".
The Security Council resolution also imposed a one-year arms embargo on CAR.