They said the bodies were found in Bukombo, around 100 kilometres (62 miles) north of the regional capital Goma after the rebels withdrew from the area.
"I'm at the scene and there are 11 corpses of civilians in front of me, some shot dead, others killed with knives," a resident told AFP over the phone. "There are no fighters among them", he said.
Other sources in Bukombo, who spoke on condition of anonymity, confirmed this toll.
One civil society leader put the number at 12, saying the rebels had fought against "self-defence groups yesterday" and left "with people they forced to carry their luggage.
"When they got to Bukombo-centre, they killed them".
The Tutsi-led M23 rebel group has seized swathes of territory and displaced about a million people in the region since re-emerging from dormancy in late 2021.
The group did not respond to a request for comment.