Al Qaeda chief Ayman al-Zawahiri has slammed the Islamic State and declared war against the Sunni jihadist group's leader Abu Bakr al-Baghdadi.

Al-Zawahiri called ISIS an illegitimate organization and criticized it for poaching fighters from other terrorist outfits into its fold. He also accused Baghdadi of sedition.

"We don't recognize this caliphate of Abu Bakr al-Baghdadi," he said in an audio message released on Wednesday, according to Reuters. 

"It's pretty interesting. Zawahiri until now has not been willing to openly condemn Baghdadi and ISIS. It highlights how deep the division is between al Qaeda leadership and ISIS. It suggests that the differences are irreconcilable. Had ISIS and al Qaeda realigned by joining forces, it would be terrible," former National Counterterrorism Center Director Matthew Olsen told ABC News.

The Al Qaeda leader, however, called for cooperation between both jihadist groups despite rivalry. He said that fight against the West could mean joining forces with ISIS.

"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," he said, according to Khaama Press.

Al-Baghdadi founded ISIS and pro-claimed his own caliphate after breaking away from Al Qaeda's Iraq wing in 2003, according to Middle East Eye.