Boko Haram extremists have claimed responsibility for a suicide bombing of a procession of hundreds of Shiite Muslims and threatened more attacks on opponents of its radical form of Islam.

Shiite leaders said at least 21 people died in Friday's attack near Nigeria's northern city of Kano.

Posting on social media Saturday as the West Africa Province of the Islamic State, Boko Haram published a photograph identifying the bomber as Abu Suleiman al-Ansari.

Its message says "with God's help, our operations will continue harvesting the infidel Rafida (Shiite) until we purge the land of their filth."

Al-Qaida has criticized Boko Haram, its former Nigerian affiliate, for indiscriminately killing fellow Muslims. In March, Boko Haram joined the Islamic State group. Boko Haram's 6-year uprising has killed some 20,000 people.