Al-Qaeda leader Ayman al-Zawahiri dismissed the Islamic State as illegitimate, but suggested Muslims should join the militant group in fighting Western forces in Iraq and Syria.
"We don't recognize this caliphate of Abu Bakr al-Baghdadi," Zawahiri said in an audiotape released on the internet September 9, referring to the IS leader.
It was not clear when the recording was made, but references to events suggest it was made at least eight months ago.
IS in the last year seized larged swaths of Syria and Iraq and declared a caliphate. As it has sought to expand its power base, it has become a greater security threat than Al-Qaeda in the strategic oil-producing region.
Despite rivalries between the two Islamist organizations, Zawahiri suggested there was still room for cooperation when it came to combating the West.
"Despite the big mistakes [of IS], 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," he said.