Mansour, a trusted deputy of longtime Taliban leader Mullah Omar whose death was confirmed last month, is taking charge as the movement faces growing internal divisions and is threatened by the rise of the Islamic State group, which is making inroads in Afghanistan.
Zawahiri’s declaration comes with Al Qaeda also facing a growing rivalry for global jihadist preeminence with IS, which has seized control of large parts of Syria and Iraq.
“As emir of Al Qaeda, I pledge to you our allegiance, following the path of Sheikh (Osama) bin Laden and his martyred brothers in their allegiance to Mullah Omar,” Zawahiri said, referring to the late Al Qaeda leader.
The recording was featured in a video that opens with images of bin Laden – who was killed by US Special Forces in Pakistan in 2011 – pledging allegiance to Omar.
The recording then plays over a picture of Zawahiri, who is believed to be in hiding in the Afghanistan-Pakistan border region.
He says that the “Islamic emirate” established by the Taliban in Afghanistan was the “first legitimate emirate after the fall of the Ottoman Empire, and there is no legitimate emirate in the world apart from it.”
He stated his opposition to any regime or organisation including Muslim ones that oppose sharia law, which he promised to implement.
And he pledged to continue “jihad until every part of occupied Muslim land is free”.
Mansour was announced as the new Taliban chief on July 31, after the movement confirmed the death of Omar, who led the Islamist movement for some two decades.
But splits have emerged in the Taliban following the appointment, with some top leaders, including Omar’s son and brother, refusing to pledge allegiance to Mansour.
Taliban spokesman Zabihullah Mujahid acknowledged the news of Zawahiri’s pledge of allegiance, but said: “We will react about it later…. We don’t want to comment on it now.”
Pakistani analyst Imtiaz Gul, an expert on Al Qaeda and the Taliban, said: “Zawahiri’s announcement is logical and true to the Islamic tradition of governance and succession, which is to say that whoever commands the majority of the Taliban should rightfully be the successor.
“This is in keeping with their political ideology. These organisations contest the idea of hereditary succession,” he added.
Just two days after the succession announcement, the late leader’s son Mullah Yakoub, and his brother, Mullah Abdul Manan, refused to pledge allegiance to Mansour, calling on religious scholars to settle the rift.
Yakoub and several other members of the Taliban’s ruling council walked out of the meeting at which Mansour was declared leader, refusing to pledge loyalty to him, a Taliban source told AFP.
The Taliban have not revealed when Omar died. The Afghan government said he passed away in Karachi in April 2013.
Official Taliban statements in the name of Omar, who had not been seen in public since the Taliban were toppled from power in 2001, were released as recently as July.
Mullah Mansour is one of the founders of the Taliban movement and is seen as a relative moderate who favours peace talks with the Afghan government.
However, he has faced powerful rivals within the Taliban who are strongly opposed to such dialogue, with some insurgents also unhappy at the thought he may have deceived them for more than a year about Omar’s death.
There was a new round of talks at the beginning of July, which was supposed to have been followed up recently but was postponed indefinitely following the news of Omar’s death.
Just days after the announcement, the head of the Taliban’s Qatar political office set up in 2013 to facilitate talks with Kabul resigned.
Tayeb Agha said he did so to maintain a “clear conscience,” adding that he would not be involved “in any kind of (Taliban) statements… and will not support any side in the current internal disputes within the Taliban.”