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Al-Qaeda chief Ayman al-Zawahiri has pledged allegiance to new Afghan Taliban leader Haibatullah Akhundzada, a who took control of the militant group last month after his predecessor was killed in a US drone strike.

The pledge comes as al-Qaeda is facing rivalry from the Islamic State group, which has also made inroads into Afghanistan where the Taliban have been waging an insurgency since they were ousted from power in 2001.

Zawahari's remarks came in a 14-minute audio and video message posted online, the US-based monitor SITE Intelligence Group said on Saturday.

"We pledge allegiance to you on jihad to liberate every inch of the lands of the Muslims that are invaded and stolen, from Kashgar to al-Andalus, from the Caucasus to Somalia and Central Africa, from Kashmir to Jerusalem, from the Philippines to Kabul, and from Bukhara and Samarkand," it quoted Zawahiri as saying.

He described the new Taliban chief as the "emir of believers" and the "legitimate" head of the Muslim caliphate.

"Allah has graced you by establishing the first legitimate emirate after the fall of the Ottoman Caliphate, and in the world there was no other legitimate emirate," he said.

Zawahiri is staunchly opposed to the IS group which in 2014 declared the creation of a "caliphate" straddling Syria and Iraq.

In Syria, al-Qaeda is represented by al-Nusra Front which is allied with other Islamist rebel groups and has been locked in fighting with IS for control of territory in the north and around Damascus.

Last August, Zawahiri made a similar pledge to Mullah Mansour, who took charge of the Taliban the previous month at a time when IS was making inroads into Afghanistan.

Akhundzada was named by the Afghan Taliban as their new leader last month in a swift power transition after officially confirming the death of Mansour.

Akhundzada, formerly one of Mansour's deputies, faces the enormous challenge of unifying an increasingly fragmented militant movement.

The drone attack in May that killed Mansour, the first known American assault on a top Afghan Taliban leader on Pakistani soil, sent shockwaves through the insurgent movement which had seen a resurgence under Mansour.

He was killed just nine months after being formally appointed leader following a bitter power struggle upon confirmation of founder Mullah Omar's death.

Observers have said that Akhundzada, a low-profile religious figure who is seen as more of a spiritual figurehead than a military commander, will emulate Mansour in shunning peace talks and intensifying attacks against the Afghan government.