Nigeria’s home-grown Boko Haram Islamic extremists have pledged formal allegiance to the Islamic State group in a message allegedly by the Nigerian insurgent leader posted on Twitter and reported by the SITE Intelligence monitoring service.

The unverified pledge posted Saturday comes as the Nigerian militants reportedly are massing in a northeastern town for a showdown with a multinational force that has dislodged them from a score of towns in recent weeks.

Boko Haram leader Abubakar Shekau makes the oath in Arabic in a tweeted message saying: “We announce our allegiance to the Caliph of the Muslims.” IS leader Abu Bakr al-Baghdadi has declared himself the caliph.

Police say four suicide bomb attacks hit the city of Maiduguri on Saturday, killing at least 54 people and wounding 143 in the heartland of Nigeria’s northeastern Islamic extremist uprising.

This is an Associated Press story.