The rapid ousting of the Western-backed Afghan government by the Taliban, who lost control of Afghanistan in the US-led invasion two decades ago, will provide jihadists with an example of how patience and careful strategy can pay off even after the defeat of IS in Syria and Iraq.

It is also especially symbolic coming just ahead of the 20th anniversary of the September 11, 2001 attacks on the United States carried out by Al-Qaeda under Osama bin Laden and planned from Taliban-ruled Afghanistan where he had taken refuge.

"The Taliban's victory will give jihadist groups worldwide a major boost. It makes them believe that they can expel foreign powers, even major military powers like the United States," Colin Clarke, director of research at the New York-based Soufan Centre think tank, told AFP.

"I expect to see a major propaganda blitz culminating on the 20-year anniversary of the 9/11 attacks. This will improve morale for jihadis from North Africa to Southeast Asia," he added.

Aymenn Jawad Al-Tamimi, a fellow at George Washington University's Program on Extremism, said that the example of the Taliban's patience would convince jihadists around the world to keep fighting, despite the existing hostility between the Afghan fundamentalists and IS.

"The Taliban's takeover of Afghanistan is something that actually emboldens jihadists everywhere although the IS is not necessarily happy about it."

"When groups see the Taliban celebrating victory, I think it convinces them that if they just keep fighting, eventually those they are fighting will collapse, whether it's in Somalia or West Africa.

"Eventually they just hope the powers backing these governments they fight will withdraw."

'Galvanises ISKP'

Al-Qaeda's propaganda arm Al-Thabat has already welcomed the Taliban takeover of Afghanistan, saying that "Muslims and Mujahideen in Pakistan, Kashmir, Yemen, Syria, Gaza, Somalia, and Mali are celebrating the liberation of Afghanistan and the implementation of Sharia within it."

The relationship between the Taliban and IS has always been more tricky and the IS branch in Afghanistan and Central Asia -- the Islamic State - Khorosan Province (ISKP) -- was set up by Taliban defectors.

But analysts say that despite ideological differences with the Taliban, IS will also profit from the collapse of the Afghan state and find Taliban-controlled Afghanistan fertile ground for its operations.

"Dr. Q", a Western expert on IS who who publishes the results of research under a pseudonym on Twitter, counted 216 ISKP attacks between January 1 and August 11, compared with 34 in the same period last year. 

"This makes Afghanistan one of the most dynamic IS provinces," he told AFP. "Everything is not directly linked to the American withdrawal, but the victory of the Taliban also galvanises the ISKP."

He said that, beyond fratricidal hatreds, there was a convergence of objectives between IS and the Taliban. 

"IS regularly states that Westerners cannot stay forever" in the region. In this regard, the triumph of the Taliban "legitimises their way of doing things".

And the chaos that could ensue in the months and years following the Taliban takeover of Afghanistan risks providing a breeding ground for all jihadist groups, who thrive on instability.

"The collapse of the Afghan army is eerily reminiscent of what we saw in Iraq in 2011," when jihadists began an insurgency that would eventually see the capture of cities like Mosul, said Clarke.

"I'm worried the same situation will unfold here in Afghanistan, with the rise of both ISIS and the resuscitation of Al-Qaeda," he said, using another acronym for the Islamic State group.