One year ago, the Islamic State (IS) presence in Yemen was about 80 people; now the group has 300 militants there, according to Aimen Dean, a former al-Qaeda insider who spoke to Reuters.
While al-Qaeda in the Arabian Peninsula (AQAP) remains the top militant network in Yemen, IS militants are asserting themselves by attacking the Houthi rebels, which have already exiled President Ali Abdullah Saleh and are fighting a Saudi-led coalition of Arab countries.
Generous funding from IS, al-Qaeda's inability to stage regular attacks on Houthis, and losses of al-Qaeda leaders in drone strikes are leading the group’s supporters to defect to IS.
IS "are supplanting al-Qaeda and presenting themselves as the credible alternative," Dean was quoted as saying by Reuters.
Ibrahim Sharqieh Frehat, a conflict resolution analyst with the Brookings Doha Center, said US strategies, including drone strikes, have helped IS to draw supporters in places like Iraq and Syria.
"The problem with the Americans is that they deal with the problems with the same tools – bombings. Weakening AQAP will only give way to Daesh to be born," he said, referring to IS by its Arabic acronym.
Katherine Zimmerman, research fellow at the American Enterprise Institute, said IS' expansion has so far not affected AQAP's dominance.
"The challenge is one that may come in the future: should ISIS come to dominate the fight against the Houthis in Yemen, AQAP may be marginalized and placed in a position where it has to respond," Zimmerman told Reuters.
AQAP seems to be losing its hold on followers in Yemen, which has been a traditional stronghold for the al-Qaeda branch since its founding in 2009.
In February, the US-based SITE monitoring center reported that a group of purported AQAP supporters had announced their defection to Islamic State in a message on Twitter.
"We announce breaking the pledge of allegiance to the sheikh, the holy warrior and scholar Sheikh Ayman al-Zawahiri… We pledge to the caliph of the believers Ibrahim bin Awad al-Baghdadi to listen and obey," they wrote, according to SITE.
Zawahiri succeeded bin Laden's as al-Qaeda leader. Baghdadi is the leader of IS.
The number of defections from al-Qaeda to IS has included leaders, as well. A Yemeni news website reported on Sunday that a senior AQAP leader, Jalal Baleidi, had switched allegiance to Islamic State.
Supporters of the groups have already begun trading jabs over social media. Dean, the former al-Qaeda insider, told Reuters he estimates that the two groups are three to six months from an armed conflict erupting between them.