In a move that may be considered the West's worst nightmare scenario, the Islamic terror group known as ISIS has accepted Boko Haram's pledge of allegiance on Thursday.

According to a report from Agence France-Presse, an audio tape from ISIS spokesman Mohammed al-Adnani seemed to confirm that the group's leader, Abu Bakr al-Baghdadi, accepted the alliance proposed by Boko Haram leader Abubakar Shekau.

"We announce to you to the good news of the expansion of the caliphate to West Africa because the caliph ... has accepted the allegiance of our brothers of the Sunni group for preaching and the jihad," Adnani said about the new alliance.

According to a report from Reuters, Adnani also congratulated "Muslims and our jihadi brothers in West Africa." He also called on Muslims to join the fight in Africa, adding that Boko Haram's pledge opened a "new door for you to migrate to the land of Islam and fight."

"We are calling you up for jihadis, go," Adnani said.

While both are Sunni Muslim extremist groups, Boko Haram is mainly based out of West Africa, while ISIS has made an Islamic "caliphate" in parts of Iraq and Syria it controls. According to AFP, ISIS has also received allegiances from jihadists in Egypt and Libya.

AFP reported that Shekau made the pledge in an audiotape attributed to him on Saturday.

"We announce our allegiance to the Caliph of the Muslims, Ibrahim ibn Awad ibn Ibrahim al-Husseini al-Qurashi," Shekau said in reference to al-Bagdadi.

According to AFP, Adnani attempted to play down attacks from Iraqi and Kurdish forces in addition to airstrikes from the coalition led by the United States. He even claimed that ISIS was growing in strength and expanding its reach.

"Our caliphate is resisting and it is advancing in the right direction," Adnani said. "We are fighting the Crusaders and the rafidah (Shiites) and day by day the Islamic State is becoming strong."

Reuters reported that Iraqi security forces and militias have fought hard against ISIS, making big gains in Saddam Hussein's home city of Tikrit on Wednesday. In Syria, Kurdish YPG fighters have cut off an important supply route to ISIS-controlled territory in Iraq.

"The State remains steadfast ... and is becoming stronger and continues to be victorious, Adnani said. "It is a mere taking back of a few villages in a war that is about attack and retreat."

According to AFP, US Security Council diplomats have been drafting a resolution to beef up a regional force fighting Boko Haram, and Iraqi forces have beaten back ISIS militants across the country.