Story highlights Recent audio messages from al Qaeda leader Ayman al-Zawahiri send mixed messages on ISIS

He criticizes ISIS' leader and its claims of a new "caliphate," but still calls for cooperation against "enemies of Islam"

But there's a caveat -- "We are not with them if they evade being ruled by the Sharia" or other actions he objects to

(CNN) After a prolonged spell of silence, al Qaeda leader Ayman al-Zawahiri has come alive with a series of audio messages. And they're raising a few eyebrows because -- in two separate messages -- al-Zawahiri has extended an olive branch to ISIS, even while describing ISIS leader Abu Bakr al-Baghdadi's "caliphate" as illegitimate.

For nearly two years, al Qaeda and ISIS have fought an unusually public battle for supremacy in the global jihadist movement. Al Qaeda disowned ISIS early in 2014 because al-Baghdadi ignored its directive to stay out of Syria. And its affiliate in Syria, Jabhat al Nusra, is viscerally hostile to ISIS.

Now al-Zawahiri has urged all jihadists in Iraq and Syria to cooperate in the face of a common enemy. And if his words are heeded -- though it's a big "if" -- they could herald a real change on the Syrian battlefield, where the Bashar al-Assad regime has benefited from perennial infighting among rebel groups.

It's not clear when al-Zawahiri's messages were recorded, but the first, posted on September 9, included references to Taliban leader Mullah Omar without acknowledging his death -- and appears to have been produced in March or April.

In it, al-Zawahiri speaks at length about how al-Baghdadi has split the Muslim community, the "sedition that al-Baghdadi and those with him seek to raise among the ranks of the mujahideen ... assigning themselves as guardians of the Muslims without consultation" in declaring the caliphate.

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