Al Qaeda leader Ayman al-Zawahiri called on the group's militants to cocoperate with Islamic State (IS) in fighting the US-led coalition in Syria and Iraq, though refused to endorse the group's self-proclaimed caliphate.

In an audiotape message released online, al-Zawahri said "We don't recognise this caliphate."

However, he called on al Qaeda fighters to put differences aside and co-operate with the rival jihadist group.

"Despite the big mistakes [of IS], if I were in Iraq or Syria I would co-operate with them in killing the crusaders and secularists and Shi'ites even though I don't recogniSe the legitimacy of their state, because the matter is bigger than that," said the Egyptian doctor, who took control of the jihadist group after Osama bin Laden was killed in 2011.

Reuters reports that references within the recording suggest that it is approximately eight months old.

IS split from al Qaeda during the early phases of the Syrian civil war that erupted in 2011, after refusing to confine its operations to Syria. IS has since seized large swaths of Syria and Iraq, eclipsing al Qaeda as the world's chief jihadist threat, with IS chief Abu Bakr al Baghdadi declaring himself Caliph or leader of all Muslims in 2014.

Isis and al Qaeda-affiliated Syrian rebel group Jabhat al Nusra have clashed in Syria. Today (9 September), al Nusra seized control of the last military base held by the Syrian army in Idlib province, northwestern Syria, after a two year siege.

Both groups have been targeted by US airstrikes, which were launched in September 2014.

Affiliates of the two groups have also established rival jihadist factions in Yemen.