The SITE monitoring group says Ansaru wrote in a cryptic message on a jihadi website that Muslims face danger from "people from their own group making them taste the same thing that they suffer from the cruel enemy."
Both Ansaru and Boko Haram were last week labelled foreign terrorist organisations by the US Boko Haram has been blamed for thousands of deaths since 2009.
Ansaru formed at the beginning of 2012 to protest the killings of Muslims by Boko Haram. Ansaru has claimed responsibility for an attack on a Nigerian military convoy to Mali and at least 10 kidnappings.