Al-Qaeda leader Ayman al-Zawahiri has pledged allegiance to the new Afghan Taliban chief in an audio message posted online.

The pledge to Mullah Akhtar Mansour was issued by al-Qaeda's media arm al-Sahab and was Zawahiri's first message since September last year.

There had been speculation about whether Zawahiri was himself dead since the death of former Taliban head Mullah Omar was confirmed last month.

Zawahiri offered his condolences.

Significant oath

Al-Qaeda and Zawahiri considered Mullah Omar to be the leader of the global jihadist movement.

This was however contested by al-Qaeda's rival, the Islamic State (IS) militant group, which announced the establishment of a caliphate last year.

In the recording, released on jihadist websites and accompanied by a still image of Zawahiri, he said: "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."

The death of Mullah Omar has been exploited by IS to try to drive defections from al-Qaeda, analysts say.

IS has been steadily expanding into al-Qaeda's turf.

Mullah Mansour's appointment as Taliban leader had been questioned by some senior members of the movement, leading to reports of divisions.

Al-Zawahiri's message - while impossible to independently verify - will be seen as significant.

Pledges of allegiance are key to a Taliban leader's legitimacy, as breaking an oath is viewed as a sin, experts say.