Pillay said she's been receiving reports the so-called anti-Balaka militia, a Christian group, is getting more heavily armed.
"We have also received reports that ex-Seleka forces have been distributing weapons to the Muslim civilian population," she said in a statement from Geneva, Switzerland.
Seleka, a Sunni rebel coalition that is formally disbanded, toppled the Central African Republic government in March. The anti-Balaka group has taken up arms against their Sunni counterparts.
"These developments are extremely worrying and should ring alarm bells around the world for sustained and urgent efforts to be taken to prevent the Central African Republic from plunging into disaster," Pillay warned.
The U.N. Security Council authorized a French military intervention to help support an African Union peacekeeping mission in CAR.
U.S. State Department spokeswoman Marie Harf said Thursday the United States is providing up to $100 million to support the multilateral intervention in CAR.
"The United States remains deeply concerned about the horrific violence committed by armed groups against innocent civilians in CAR," she added.