Al Qaeda leader Ayman al-Zawahiri, in an online audio message, pledged allegiance to the recently appointed head of the Afghan Taliban, Mohammad Akhtar Mansour, in a move that could bolster his accession after the death of Taliban founder Mullah Mohammad Omar.
"We pledge our allegiance ... (to the) commander of the faithful, Mullah Mohammad Akhtar Mansour, may God protect him," said Ayman al-Zawahiri, believed to be hiding in a border area between Afghanistan and Pakistan that is a militant bastion.
The authenticity of the recording could not be immediately verified, but it had all the stamps of an al Qaeda video.
Divisions within the Taliban insurgent movement have emerged since the news last month of the death of Mullah Omar.
The swift announcement that Mansour, Omar's longtime deputy, would be the new leader has riled many senior Taliban figures, and Omar's family said this month that it did not endorse the move.
Mansour's position could be shored up by the vote of confidence by al Qaeda, the global militant group that has maintained ties with the Taliban for almost two decades since the tenure of its founder and late leader Osama bin Laden.
"As leader of the al Qaeda organisation for jihad, I offer our pledge of allegiance, renewing the path of Sheikh Osama and the devoted martyrs in their pledge to the commander of the faithful, the holy warrior Mullah Omar," Zawahiri added.
Reiterating support for the Taliban is also a tacit rejection of Islamic State, the new ultra-radical Sunni Muslim movement that is ensconced in Iraq and Syria and has gained the support of a few Afghan insurgent commanders.
Al Qaeda is being challenged by Islamic State for leadership of the global jihadist movement, as determined backers of IS have cropped up in Libya and Yemen this year.
Al Qaeda was set up by Arab guerrillas who flocked to Afghanistan to fight Soviet occupation forces in the 1980s. It thrived under the Taliban's 1996-2001 rule in Afghanistan before the US invasion that followed al Qaeda's September 11, 2001 attacks on New York and Washington drove both groups underground.