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"Jabha East Africa" announced its alliance on Thursday as it praised a "new era" in the region, giving an oath of allegiance to Abu Bakr al-Baghdadi as the "rightful Khalifa [Caliph] of all Muslims", the group said in a statement:

"To pledge bayah [an oath of allegiance] to Caliph Abu Bakr al-Baghdadi is freedom for the mujahideen [fighters] in East Africa and opportunity to wage jihad according to the Sunnah against the enemies of Allah."

The background of the group is unknown, however reports suggest it comprises of defectors from al-Shabab.

"We in Jabha East Africa are advising all East Africans to leave al-Shabab and their sponsor groups, like Al-Muhajiroun, Al-Hijra and Ansar Islam," the statement said.

"Like Al-Shabab the sponsor groups have not understood the binding obligation of the Khalifah [caliphate].

"We are telling the mujahideen in East Africa that al-Shabab has now become a psychological and physical prison." 

The news comes as the head of US forces in Africa warned the number of IS fighters in Libya had doubled to up to 6,000 in as little as a year.

IS has exploited the turmoil in Libya since the overthrow of dictator Muammar Gaddafi five years ago, raising fears that it is establishing a new stronghold on Europe's doorstep.

The IS group last year seized control of Gaddafi's coastal hometown of Sirte and has been fighting to expand to other areas.

Rodriguez said that Libyan militias "are contesting the growth of ISIS in several areas across Libya"m using another acronym for the militant group.

"In the east, in Benghazi and Derna, they have fought back against the Islamic State and made it much tougher for them to operate."