Violent clashes led to the deaths of about 100 people, with many others fleeing their homes, a U.N. statement said.
The country experienced a resumption of attacks by anti-government Seleka rebels this month.
"I am gravely concerned that this rapid deterioration of the situation could lead to further displacement," said U.N. Undersecretary-General for Humanitarian Affairs Valerie Amos.
Members of the U.N. staff in the city of Bangui "say the situation is very tense this morning," Office of U.N. High Commissioner on Human rights spokesman Adrian Edwards said Friday. "We are hearing worrying reports of sectarian and revenge attacks between neighbors throughout Bangui. A local UNCHR worker was attacked in his home last night and the assailants took away and killed his 24-year-old nephew."
The UNCHR said nearly 400,000 people have been displaced since the violence resumed, and another 69,800 have fled to neighboring countries.
France began its military intervention in Central African Republic after receiving approval by the U.N. Security Council, French Defense Minister Jean-Yves Le Drian said.
The number of French troops in the African country will be boosted from about 650 to 1,200 during the weekend, Radio France Internationale reported Friday.
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