KABUL (AFP) - The Afghan Taleban Sunday published a descriptive biography of their "charismatic" supreme leader Mullah Omar, in a surprise move apparently aimed at countering the creeping influence of the Islamic State in Iraq and Syria (ISIS) within insurgent ranks.

The Taleban have reportedly seen defections to ISIS in recent months, with some insurgents expressing their disaffection with the one-eyed warrior-cleric who has not been seen since the 2001 US-led invasion of Afghanistan.

The biography, posted just after midnight on the Taleban's main website to commemorate Omar's 19th year as supreme leader, described him as being actively involved in "jihadi activities", dispelling speculation that he had died.

Despite being "regularly tracked by the enemy, no major change and disruption has been observed in the routine works of (Omar) in... organising the jihadi activities as the leader of the Islamic Emirate," it said.

"He keenly follows and inspects the... activities against the brutal infidel foreign invaders."

Lionising the "charismatic personality", the biography also contained several anecdotes of battlefield valour and described the RPG-7 grenade launcher as Omar's "preferred weapon of choice".

The withdrawn, remote figure has not made a public appearance since the 2001 invasion, and has hardly ever been photographed.

The US State Department - which has a US$10 million (S$13.5 million) bounty on his head - only describes him as a tall male with a shrapnel wound to the right eye.

The Taleban's surprise move to release his richly-detailed biography, even describing his personal and family life, took security analysts by surprise.

"The Taleban have posted Omar's biography for several strategic reasons - the most important of which is to counter Daesh influence in their ranks," said Ahmad Sayedi, an expert on the Taleban, alluding to the Arabic abbreviation for ISIS.

"This announcement is also meant to show that Omar is alive and well and still in control as the supreme leader of the Taleban."

In the past 13 years, Omar has stayed completely out of the public eye amid growing power struggles within the Taleban and fears of ISIS's influence in their ranks as an ideological rival.

The Afghan government has also raised the ominous prospect of the ISIS making inroads into the country, though the group that has taken over swathes of territory in Iraq and Syria has never formally acknowledged having a presence in Afghanistan.

In February a US-led Nato drone strike killed a former Taleban commander and a Guantanamo detainee suspected of links to ISIS in the volatile southern province of Helmand.

Mullah Abdul Rauf Khadim, who led around 300 men, had reportedly defected from the Taleban to join ISIS.

The whereabouts of Omar remain a mystery but he is believed to be leading the Taleban insurgency in Afghanistan from a hiding place somewhere in Pakistan.