Three of the dead were said to be children, according to spokesman Mike McCusker. He said doctors had described gory scenes after gunmen ambushed a convoy of refugees, firing a rocket-propelled grenade to halt the vehicles before attacking with firearms and machetes.
Mr McCusker told reporters: "Our doctor said there was blood everywhere, just pouring out of people like tap water."
The attack in the remote north-west of the country outside the town of Bouar illustrates how African and French peacekeepers are not reaching remote areas where violence goes unreported, the British charity's country director Robert Lankenau said.
More than 1000 people have died in the Central African Republic since December and nearly one million have been forced from their homes since a rebel leader seized power last year.