(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 cannot 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 cannot 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 cannot get to Iraq or Syria . 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 .