Al Qaeda Leader Ayman al-Zawahiri issued a new audio message on Thursday pledging his allegiance to the new Afghan Taliban chief Mullah Mohammad Akhtar Mansur.

"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 in a recorded statement released on Jihadists websites and social media accounts, according to BBC. 

"We pledge our allegiance to our lord, leader of the faithful, Mullah Muhammad Akhtar Mansoor, may God protect him," he said, according to EFE.

The U.S. counterterrorism officials said there was no reason to believe Zawahiri's audio message is not authentic, reported Voice of America. They, however, are still studying the message.

Al Qaeda chief, who is under pressure due to continuous rise of Islamic State group, refused to accept ISIS's self-claimed Caliphate as legitimate emirate.

He said that the Afghan Taliban's Islamic emirate was the first legitimate emirate after the fall of the Ottoman Empire, and there is no legitimate emirate in the world apart from it, according to AFP.

Taliban's new leader Mullah Mansur welcomed al-Zawahiri's pledge of allegiance.  "I... accept the pledge of allegiance of the esteemed (Zawahiri)... and thank him for... his pledge and pledge of all mujahideen under him," Mansur said in a statement Friday, according to Samaa TV.

Zawahiri's pledged of allegiance came just few weeks after Afghan Taliban announced death of it's leader Mullah Omar. The militant group's new leader Mullah Mansur is facing tough opposition from a section of Taliban. Al Qaeda's pledge of allegiance to Mansur could help him to gain legitimacy within Taliban.