ABUJA, Nigeria - The Islamic State has reportedly accepted a pledge of allegiance from Nigeria's Boko Haram militant group, paving the way for a dangerous terror alliance that extends the Islamist caliphate to West Africa.

In a 28-minute audio message released Thursday by the Islamic State's al-Furqan media outlet, the group's spokesman Abu Mohammed al-Adnani claims that the caliphate had been expanded to West Africa.

On Saturday, Boko Haram leader Abubakar Sheka posted an audio recording online that pledged allegiance to the Islamic. The group wants to establish an Islamist state in parts of Nigeria which they say will be strictly governed as per the Islamic code called Sharia.

Adnani had previously urged fighters from around the world to migrate and join Boko Haram. The new agreement between the militant groups mirrors the steps taken by other Sunni jihadist groups in Libya and Egypt.

The announcement of the merger comes as the most ruthless terror groups in the world have been weakened by military pressures inflicting on them sustained setbacks on battlefields.

In the audio message attributed to Islamic State caliph Abu Bakr al-Baghdadi, the spokesman says that the caliphate has expanded to western Africa and congratulated "our jihadi brothers" in Nigeria.

Adnani said that now "no one can stand in its path"

"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."

Adnani encourages people in the message to join fighters in Africa if they cannot make it to Iraq or Syria.

Boko Haram has been hit by a multinational force and its fighters have been chased away from its strongholds in northeastern Nigeria.

But the most immediate advantage from allying with the Islamic State is a propaganda and possible recruiting boost for Boko Haram.

The Nigeria group gets an expanded social media presence with the Islamic State's help. The Islamic State in turn gains the support of its largest affiliate yet.

Earlier, there were indications that the Islamic State is helping the Boko Haram with its propaganda machinery. Its new Twitter account, increasingly slick and more frequent video messages, and a new media arm all were considered brainchild of the Islamic State.

Jacob Zenn, a terror expert who lives in Nigeria, told CNN that 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.

The Islamic State "gets more international legitimacy as a global caliphate", he said.