Members of ISIS on Thursday reportedly released an audio recording in which the group accepts Boko Haram's pledge of allegiance to the Islamic State, Reuters reported.
"Our caliph, God save him, has accepted the pledge of loyalty of our brothers of Boko Haram so we congratulate Muslims and our jihadi brothers in West Africa," said Islamic State spokesman Mohammed al-Adnani, encouraging those who could not fight in Syria or Iraq to join Boko Haram in Africa.
"Our caliphate is resisting and it is advancing in the right direction. We are fighting the Crusaders and the rafidah (Shiites) and day by day the Islamic State is becoming strong."
The message was shared by the Islamic State on social media, according to the SITE Intel Twitter account, which monitors extremist groups.
Breaking: Speech by #ISIS spokesman Abu Muhammad al-Adnani lauds #BokoHaram for pledge, threatens Jews & Christians
On Saturday, Boko Haram leader Abubakar Shekau announed that they were joining with ISIS, and "pledged our allegiance to the Caliph of the Muslims."
Boko Haram had previously mentioned ISIS in videos, but had not formally announced an alliance with the terrorist organization until last week.
Although facing air strikes from the United States and its allies, the Islamic State has seized large areas of Iraq and Syria over the past year, deeming it an Islamic "caliphate." Boko Haram has claimed more than 13,000 lives and left 1.5 million people homeless during its six year insurgency in Nigeria.
This story is developing. Check back for details.