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Egyptian doctor al-Zawahiri said in the message: “We have endured a lot of harm from Abu Bakr al-Baghdadi and his brothers, and we preferred to respond with as little as possible, out of our concern to extinguish the fire of sedition.

Al-Zawahiri accused al-Baghdadi of "sedition" for last year calling on the world's Muslims to "obey" him as the head of the caliphate.

Ayman al-Zawahiri, who replaced Osama bin Laden four years ago, said he had “no choice” but to destroy the competiton, in a angry rant at ISIS chief Abu Bakr al-Baghdadi.

Britain needs to destroy ISIS NOW by joining forces with PUTIN and...

“But Abu Bakr al-Baghdadi and his brothers did not leave us a choice, for they have demanded that all the mujahideen reject their confirmed pledges of allegiance, and to pledge allegiance to them for what they claim of a caliphate."

The imminent showdown could play into the hands of counter-terrorism operations – who could exploit conflict as the two jihadi groups are pitted against each other.

Al-Baghdadi declared himself as leader of the caliphate in a rare public speech in Mosul, Iraq, last year.

He said: "I am the wali [leader] who presides over you, though I am not the best of you, so if you see that I am right, assist me.

“If you see that I am wrong, advise me and put me on the right track, and obey me as long as I obey God in you.”

Zawahiri said "everyone was surprised" by al-Baghdadi's declaration, which he had done "without consulting the Muslims".