( CNN ) In an audio message purportedly from an ISIS spokesman , the group announced that a pledge of allegiance from Nigerian-based Boko Haram has been accepted by ISIS leader Abu Bakr al-Baghdadi . The 28-minute message , which can not be independently authenticated by CNN , was posted online by ISIS supporters . The message says that the caliphate , or Islamic State , has expanded to western Africa and congratulated `` our jihadi brothers '' there . The spokesman , Abu Mohammed al Adnani , encourages people to join fighters in Africa if they can not make it to Iraq or Syria . Boko Haram 's leader , Abubakar Shekau , announced in an audio message last week that the Islamist terror group was going to ally with ISIS . Jacob Zenn , a terror expert who lives in Nigeria , told CNN on Saturday the alliance would make sense for both groups . `` Boko Haram will get legitimacy , which will help its recruiting , funding and logistics as it expands , '' Zenn said . `` It will also get guidance from ISIS in media warfare and propaganda . Previously Boko Haram was a sort of outcast in the global Jihadi community . Now it is perhaps ISIS 's biggest affiliate . `` ISIS gets more international legitimacy as a global caliphate . '' Boko Haram , whose name translates as `` Western education is sin , '' has been waging a yearslong campaign of terror aimed at instituting its extreme version of Sharia law . Boko Haram 's tactics have intensified in recent years , from battling Nigerian government soldiers to acts disproportionately affecting civilians -- such as raids on villages , mass kidnappings , assassinations , market bombings and attacks on churches and unaffiliated mosques . Much of this violence has taken place in Nigeria . But neighboring countries , such as Cameroon and Chad , have also been hit increasingly hard . What Boko Haram 's pledge of allegiance to ISIS means . CNN 's Paul Cruickshank contributed to this report .
Spokesman encourages potential fighters to go to Africa if they can not get to Syria or Iraq . Abu Bakr al-Baghdadi 's spokesman congratulated `` our jihadi brothers '' in western Africa . Boko Haram 's leader had extended his pledge to be an ally in an audio message last week .