They may be separated geographically by more than 2,500 miles but the Islamic State appears to have warmly welcomed a weekend pledge of allegiance from Nigerian Islamic militant group Boko Haram.

An audio recording that purportedly features Islamic State spokesperson Mohammed al-Adnani emerged on Friday. "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," it stated.

The speaker goes on to encourage Muslims in West Africa to join militant Islamist groups.

"If you want to save your blood and money and live in safety from our swords... you have two choices: either convert or pay jezyah [non-Muslim tax]," Andani apparently said. "[Otherwise] you will soon bite your fingers with remorse."

In the initial outreach — an audio message posted by Boko Haram on Saturday — the group's leader Abubakar Shekau appeared to declare: "We announce our allegiance to the caliph... and will hear and obey in times of difficulty and prosperity." He added: "We call on Muslims everywhere to pledge allegiance to the caliph."

VICE News has not been able to independently verify the identity of the voices in the recording.

Both the Islamic State and Boko Haram have separately declared an aim to establish Islamic caliphates. The somewhat successful realization of these goals can be seen in the Islamic State's attempts to create a state in Syria and northern Iraq, and in Boko Haram's territorial gains in northeastern Nigeria.

Boko Haram currently controls around 10,000 square kilometers (6,200 sq miles), though this territory is under constant attack from the Nigerian army as well as forces from neighboring countries. In February, a US politician estimated that the Islamic State controls an area the size of Belgium.

Concerns were sparked earlier in March when a Boko Haram video, titled Harvest of Spies, appeared to mimic the style of similar Islamic State productions. The footage appeared to show militants beheading two men — identified as Dawoud Muhammad and Muhammad Awlu.

Other Islamic groups have clamored to be recognized by the Islamic State over the past year, ever since it began receiving large amounts of attention for its brutality and barbaric murders. According to IntelCenter, a privately owned company that provides "counterterrorism intelligence services" to intelligence, military and law enforcement communities, at least 31 groups have pledged allegiance or support to the Islamic State since August.

In September last year, a group known as the Soldiers of the Caliphate in Algeria swore allegiance to self-professed Islamic State leader Abu Bakr al-Baghdadi. Just one week later, the militants kidnapped French citizen and mountain guide Herve Gourdel, threatening to kill him if France refused to halt airstrikes against the Islamic State. Gourdel was beheaded several days after his capture, and his body was discovered near Akbil, northern Algeria, in January.

Other groups who have publicly declared an association include the Islamic Movement of Uzbekistan, and the Supporters for the Islamic State in Yemen.

Al-Baghdadi has previously declared himself the "leader of Muslims everywhere," but the International Business Times reported that the allegiance application is "strict." The process involves a public declaration, proof that the group can cooperate with other regional Islamist militant groups, the nomination of leadership, and the commission of an act of brutality as a symbol of loyalty.