Over half those killed in Ntumbo were children, and local media report that several victims were burned alive.
Cameroon's government, which has been fighting separatists in the region for three years, denied involvement in the attack.
James Nunan, an official from the UN's humanitarian co-ordination agency Ocha, told the BBC that a pregnant woman was among those killed.
Fourteen children, including nine under the age of five, were also among the dead, he said.
"Whichever group has done this has threatened that there will be more violence ahead," he said. "The people we've spoken with are extremely traumatised and didn't expect this."
In a statement, one of the country's main opposition parties - The Movement for the Rebirth of Cameroon - blamed "the dictatorial regime" and the head of Cameroon's security forces for the attack.
An army official described the allegations as "false" when asked about the incident by AFP news agency.
The conflict has been centred in north-western Cameroon, where most of the population speaks English because of old colonial ties to Britain.
More than 3,000 people have died since the outbreak of fighting and at least 70,000 people have also been displaced from their homes.
The government says separatists have killed dozens of civilians and security forces, but no official figures exist for civilians and separatists killed by government forces. CNN
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