"According to information we have received, 27 Muslims were killed by self-defence militias, known as anti-Balaka, in the village of Bohong ... on Thursday," the UN's rights agency UNHCR's spokesperson Ravina Shamdasani told reporters in Geneva.
A "vicious cycle of attacks and reprisals" is underway in the country, she said.
Speaking to French soldiers after arriving in Bangui on Friday, Le Drian said that the "spiral of confrontation has seriously worsened", bringing a risk of humanitarian crisis.
French military sources say that Muslim Seleka militias have largely been disarmed but now armed Christian anti-balaka groups need to be contained.
About 110,000 people who have fled recent violence are living in camps in Bangui, while tens of thousands are living in camps around Bossongoa in the north-west.
The CAR is a "country adrift", Le Drian said, warning that there is a risk of "anarchy" that could destabilise the whole region by attracting criminal and terrorist groups.
Le Drian was to meet interim President Michel Djotodja, who was brought to power by the Seleka but has been unable to prevent some of them going on the rampage.
He will also meet the the commander of the French contingent in Bangui, General Francisco Soriano, before going to neighbouring Chad, where he will meet President Idriss Deby.
French President François Hollande, who visited the CAR on Tuesday after two soldiers were killed, will attend Monday's ceremony in homage to them, Le Drian said.
On a visit to Brazil on Friday, Hollande called for a permanent European fund to finance emergency interventions in crises such as that in the CAR.
He is to propose the move at a European summit on 19-20 December.