"Boko Haram fighters moved into Ashigashya" overnight Monday, where they slaughtered three people in front of a church, a Cameroon police source told AFP on condition of anonymity.
Almost 500 Nigerian soldiers fled the Nigerian border cities of Ashigashyia and Kerawa over the weekend to take refuge from Boko Haram militants on Cameroonian territory.
Nigeria's military dismissed the claims and maintained that the troops were "charging through the borders in a tactical manoeuvre" when they found themselves on Cameroonian soil.
Straddling the border between Cameroon and Nigeria, Ashigashya is bisected by a small river which marks the border between the two countries.
The police source said Boko Haram is now trying to take the Cameroonian side of the city.
"Cameroonian gendarmes fled the brigade to sleep at school in the city. When they tried to return to the police this morning, they were greeted by fire from Boko Haram" from the Nigerian side, said the police officer.