Al Qaeda leader Ayman al-Zawahiri said in a newly released audio message that while he doesn't consider the Islamic State's declaration of a caliphate legitimate, he's not opposed to cooperating with the militants in the fight against the U.S.-led coalition in Iraq and Syria.

"We don't recognize the caliphate of Abu Bakr al-Baghdadi," he says in an audiotape released by al Qaeda's official media wing. He also tells Muslims they are not obligated to join it.

"Despite the big mistakes [of Islamic State], if I were in Iraq or Syria I would cooperate with them in killing the crusaders and secularists and Shiites even though I don't recognize the legitimacy of their state, because the matter is bigger than that," he explains.

The recording could have been made up to eight months ago, according to Reuters. This is only the second public message from al-Zawahiri in the last year. The first, released in August, is a message in which al-Zawahiri pledges allegiance to the Taliban's new leader, Mullah Akhtar. Al-Zawahiri had recognized the Taliban's former leader, Mullah Omar, as caliph. He found himself in hot water when al Qaeda released proclamations despite confirmation that Mullah Omar had died.

Al-Zawahiri assumed leadership of al Qaeda in June 2011 in the wake of the U.S. killing of Osama bin Laden. He "sees himself as the legitimate heir to bin Laden," Brookings Institution's Bruce Riedel told The Huffington Post. "Since Baghdadi won't [accept this] I don't see a reconciliation but this tape suggests for the first time that they can cooperate on the ground despite differences. That's bad news for the Assad regime."

A coalition of Western countries has been waging an airstrike campaign to dismantle the Islamic State in Iraq and Syria. It has carried out over 6,500 strikes in the last year.