Al Qaeda kingpin Ayman al-Zawahri snubbed the Islamic State, its leader Abu Bakr al-Baghdadi and its self-declared caliphate as illegitimate, but said al Qaeda fighters would join the group, also known as ISIS, in fighting the U.S.-led coalition in Iraq and Syria if possible. In an undated audiotape placed on the Internet, Zawahri did not detail his intentions but hinted at cooperation between the two Sunni Muslim extremist movements despite their rivalry, Reuters reported Wednesday.

"We don't recognize this caliphate,” Osama bin Laden's successor said in the recording. "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 Shi'ites even though I don't recognize the legitimacy of their state, because the matter is bigger than that.”

It was not immediately clear when the audiotape was created, but references to events suggest it is at least eight months old, according to Reuters.

Ayman al-Zawahri An employee at the Common Anti-Terror Center of the German Interior Ministry in Berlin monitors a broadcast by al Qaeda leader Ayman al-Zawahri on Al Jazeera in 2005. Photo: Sean Gallup/Getty Images

Zawahri, who became the leader of al Qaeda in May 2011 following the U.S. killing of Osama bin Laden, previously denounced the Islamic State group. U.S. counterterrorism officials have long worried about a “potential competitive dynamic” between extremist factions in attacking the United States as al Qaeda splits and morphs into various affiliates and branches. This splintering created ISIS, which has seized swaths of Iraq and Syria since 2013 and is vying for recruits among young militant Muslims with Zawahri’s core al Qaeda group, the Washington Post reported.

ISIS on Monday captured the last major oilfield under the Syrian government’s control northwest of the ancient city of Palmyra, which the Sunni Muslim militants took in May. The Islamic State group’s success against Syrian President Bashar Assad, his minority sectarian government, the Shi'ite Muslim majority in Iraq and the Western-led coalition has fueled the rivalry with al Qaeda. But Zawahri’s audio message suggests he would not oppose merging with ISIS on the battlefield in Iraq and Syria in a shared campaign.