Belmokhtar's Al-Murabitoun group, which is active in north Africa, was linked to Al-Qaeda, but the recording attributed to one of its leading members said it was now aligning itself with the IS organisation.
"The Al-Murabitoun movement pledges its allegiance to the caliph of Muslims Abu Bakr al-Baghdadi (IS leader), thereby banishing divisions and dissent within the (Muslim community)," according to the recording.
The Mauritanian news agency, which regularly publishes statements by jihadists, had earlier identified the voice in the recording as that of Adnan Abu Walid Sahraoui. It also claimed that Sahraoui was now the new leader of the group.
But an agency spokesman later told AFP that Sahraoui did not personally read out the message, although it was read out by someone on his behalf.
The official said it was clear that Sahraoui had become the group's leader since only the chief has the right to sign such a declaration.
An AFP journalist familiar with the militant also said the voice did not correspond to that of Sahraoui, who could not be reached immediately for comment.
In the recording, the militant also called on "all jihadist movements to pledge allegiance" to Baghdadi, self-proclaimed caliph of IS, in order to "speak with one voice".
Al-Murabitoun's apparent decision to align with the IS group came just two months after a similar move by Boko Haram -- the jihadist militants wreaking havoc in northern Nigeria.
IS sprang to worldwide prominence in June last year when it overran large parts of Iraq and Syria, declaring a "caliphate" in the territory under its control.
Several local jihadist groups quickly affiliated themselves, and IS has since received pledges of fealty from extremist outfits from as far afield as Algeria, Afghanistan and Indonesia.
Although the jihadist group has since been pushed back militarily by US-led air strikes and a counter-offensive by Iraqi security forces and militias, IS remains a potent draw for would-be recruits.
The Al-Murabitoun group was born out of a merger between Belmokhtar's "Signatories in Blood" and MUJAO -- one of the jihadist groups that seized control of northern Mali in early 2012 to 2013.
Its leader was Belmokhtar, who is also a former chief of Al-Qaeda in the Islamic Maghreb (AQIM) and is wanted for having allegedly masterminded a siege in January 2013 of an Algerian gas plant in which 38 mostly Western hostages were killed.
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But if Sahrouai had indeed taken over, it could further spark speculations over Belmokhtar's fate.
The jihadist was thought to have been killed in Mali, but security sources had told AFP last year that he had moved into Libya and remained active.
Sahrouai meanwhile, is not an unknown jihadist -- having often spoken for both MUJAO and Al-Murabitoun particularly in claiming responsibility for attacks and kidnappings in northern Mali.
- 'Losing traction' -
Divided among rival armed factions, plagued by drug trafficking and infiltrated by jihadist groups, Mali's desert north has struggled to maintain stability since the west African nation gained independence in 1960.
The country descended into chaos in 2012 when an insurgency by Tuareg rebels led to a coup in the capital Bamako. Jihadists such as MUJAO then overpowered the Tuareg to seize control of Mali's northern desert.
A French-led military operation launched in January 2013 drove the extremists into the bush but the Tuareg rebels and Islamist militants remain active throughout the northeast.
Al-Murabitoun had claimed responsibility for the first attack against Westerners in Bamako on March 7, targeting a bar that was popular with expatriates. The attack left five dead, including three Malians, a French national and a Belgian.
The group also carried out a suicide attack on April 15 against the Nigerian contingent of the UN mission in Mali, killing two civilians.
However, a Mauritanian expert on jihadist movements in North Africa, Isselmou Ould Salihi, said Thursday's development could be seen as a sign that the jihadist group is "losing traction" and raises the question about Belmokhtar's position.
"The rise in power of (Sahraoui), who signed the statement in place of his official boss, and the shift in the ideology of the group in quitting Al-Qaeda for IS is the first suggestion that the group is losing traction," he said.