However, Al-Shabab has played down previous requests by the Nigeria-based which already joined the Islamic State group as well as calls by ISIS itself.

In an attempt to silence dissenting voices advocating for ISIS’s allegiance, Al-Shabab fighters have detained several foreign fighters it accused of trying to switch sides last month. The unidentified fighters were arrested after the group’s leadership called for its followers to stick to their allegiance to Al-Qaeda.

Security analysts said that Al-Shabab may not be ready to join ISIS by dropping Al-Qaeda which unlike ISIS sent fighters and logistics support to the group since it rose to prominence in 2007.

However, the quick rise and prominence of ISIS is a worrying trend for Al-Qaeda as Islamic State’s leadership has started building alliance with Al-Qaeda affiliates, including the Nigeria-based group, Boko Haram which has declared its allegiance to ISIS last year.

The Islamic State group's gains over the past year have been sizeable. For nearly two decades, al-Qaida was unchallenged as the world's most prominent terrorist organization. But IS has stormed forward to rival it — and even surpass it in places, according the British newspaper, Guardian.

Perhaps more importantly, the Islamic State group has a dynamism and fervor that has seemed to fade for al-Qaida. The IS declaration of a "caliphate" in Iraq and Syria inspired a stream of thousands of foreign fighters to join it and earned it pledges of allegiance by individual militants around the region. The group's notorious brutalities — everything from beheadings to enslavement of women from religious minorities — are seen by its supporters as proof of its purity and refusal to compromise on what it considers "God's law.", according to the Military and defence site, The Insider.

ISIS leader Abu Bakr al-Baghdadi