Ayman Al-Zawahiri
Ayman Al-ZawahiriReuters

The leader of the Al-Qaeda terrorist group, Ayman al-Zawahiri, on Wednesday said that rival group Islamic State (ISIS) and its leader Abu Bakr al-Baghdadi are “illegitimate”, but also said his followers would join them in fighting the Western-led coalition in Iraq and Syria if possible.

Speaking in an audiotape on the internet and quoted by the Reuters news agency, Zawahiri said, "We don't recognize this caliphate."

It was not clear when the recording was made but references to events suggest it was made at least eight months ago.

ISIS has declared a caliphate in large parts of Iraq and Syria under its control and sought to expand its power in the Middle East.

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 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 said, according to Reuters.

Zawahiri did not elaborate, but his comments raised the possibility he could seek cooperation with the movement that has in many ways eclipsed Al-Qaeda.

While Al-Qaeda has specialized in high profile bombings, ISIS seems bent on seizing and holding territory in its quest to create a self-sustaining caliphate.

Despite fierce fighting in Syria between ISIS and a coalition of powerful jihadist groups which included Al-Qaeda’s Syria affiliate Al-Nusra Front, Al-Nusra and ISIS had temporarily agreed to work together several months ago.

However, by June this year that alliance seemed to have ended, when ISIS jihadists beheaded 12 men from rival Syrian rebel movements. Three of those killed in the new video were from Jaysh al-Islam, one of the main rebel groups in the Damascus area, and a fourth from Al-Nusra Front.