Abubakar Shekau has said he still leads Boko Haram, exposing the biggest rift yet among Nigeria's Islamic insurgents and possibly paving the way for a break from Islamic State (IS).
IS on Wednesday announced that Abu Musab Barnawi is the new leader of its so-called West Africa province.
But an audio speech on Thursday purporting to be from Shekau criticises Barnawi.
It also says IS leader Abu Bakr Baghdadi did not respond to letters explaining that Barnawi is "an infidel" preaching "false creeds".
Shekau's speech highlights ideological differences with Barnawi, who has promised to end attacks on mosques and Muslim markets - attacks that had become a hallmark of Boko Haram.
Shekau in March 2015 pledged allegiance to Baghdadi and gave IS its first franchise in sub-Saharan Africa.