One attack targeted a wedding party.
The witnesses said eight victims died when extremists attacked a wedding reception at Tashan-Alede village in Borno state late Saturday night, and four in neighboring Kwajffa village yesterday.
A security official confirmed the attacks but said they did not yet have casualty figures. All spoke on condition of anonymity for fear of reprisals.
The leader of a 4-year-old Islamic uprising that has killed thousands in northeast Nigeria recently threatened to target Christians.
Village attacks have become common with many more Muslims than Christians killed this year.