The Taliban has admitted covering up the death of long-time leader Mullah Omar for two years, saying it wanted to keep it secret until foreign forces ended their fight against the militants.

The group confirmed in July that Omar had died but did not say when, deepening internal divisions.

Many insurgents accused the leadership of keeping them in the dark while issuing statements in his name.

The admission of a cover-up was buried in a lengthy biography of new Taliban chief Mullah Akhtar Mansour, apparently published to improve his image and quell the growing internal rancour over his appointment.

Who was Mullah Omar? Most senior leader of the Taliban

Most senior leader of the Taliban Born in 1960 in the village of Chah-i-Himmat in Kandahar province

Born in 1960 in the village of Chah-i-Himmat in Kandahar province An ethnic Pashtun

An ethnic Pashtun A prayer leader and teacher in younger days

A prayer leader and teacher in younger days Married Osama bin Laden's eldest daughter in 1998

Married Osama bin Laden's eldest daughter in 1998 Head of state in Afghanistan from 1996 to 2001

Head of state in Afghanistan from 1996 to 2001 Led a hardline fundamentalist regime drawing international outrage

Led a hardline fundamentalist regime drawing international outrage US had a $US10 million bounty on his head

US had a $US10 million bounty on his head Said to have only left Afghanistan once to visit Pakistan

Said to have only left Afghanistan once to visit Pakistan Official Taliban biography describes him as "charismatic"

Official Taliban biography describes him as "charismatic" Has not been seen in public since 2001

Has not been seen in public since 2001 During meetings in July 2015, the Taliban confirmed Mullah Omar had died

Posted on the Taliban website in five languages, the biography acknowledged Omar died in April 2013 — as was first claimed by Afghan intelligence.

"Several key members of the supreme leading council of the Islamic Emirate (Taliban) and authentic religious scholars together decided on concealing the tragic news of passing away of [Omar] ... and keep this secret limited to the very few colleagues who were already informed of this incorrigible loss," the biography, which ran to nearly 5,000 words, said.

"One of the main reasons behind this decision was ... that 2013 was considered the final year of power testing between the mujahedeen and foreign invaders who ... had announced that at the end of 2014, all military operations by foreign troops would be concluded."

NATO ended its combat mission in Afghanistan last December and pulled out the bulk of its troops, although a 13,000-strong residual force remains for training and counter-terrorism operations.

Omar, dead or alive, was seen as a unifying figurehead who observers said kept the fractious movement from splintering as the rival Islamic State (IS) group began making inroads into Afghanistan.

The Taliban had continued as recently as July to release official statements in the name of Omar, lionised as a "commander of the faithful" who commanded the loyalties of militants across the region.

"The Taliban had to conceal the death of a leader who had an uncanny power to rally militants around him," Kabul-based military analyst Atiqullah Amarkhil said.

"His name was enough to prevent the Taliban's disintegration."

But the Taliban apparently came under pressure to confirm his death after the Afghan spy agency said he died two years ago in a Karachi hospital following recent reports he had been killed.

AFP