The Taliban Monday admitted covering up the death of longtime leader Mullah Omar for two years, saying they had 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.

Posted on the Taliban website in five languages, the biography acknowledged that 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," said the biography, which ran to nearly 5,000 words.

"One of the main reasons behind this decision was... that 2013 was considered the final year of power testing between the mujahideen 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 say 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.

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"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 told AFP.

"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.

- 'Jihadi acumen' -

The Taliban biography of Mansour lauded his "jihadi acumen", describing him as a pious warrior who is "naturally bequeathed with unique leading and guiding capabilities".

It said his biggest achievement was revamping Afghanistan's fledgling air fleet and airports as the civil aviation minister during the Taliban's 1996-2001 rule over the country.

The biography was also peppered with details about his frugal lifestyle, saying "he likes and wears loose, neat and clean clothes... (and) dislikes and avoids extravagance".

It added that he "speaks less and tries to listen more to other people".

Mansour's contentious rise to power has triggered a power struggle within the Taliban, which has suffered a string of defections to IS.

The biography said Mansour's appointment was "totally legitimate".

But some top leaders including Omar's son and brother have refused to pledge allegiance to Mansour, saying the process to select him was rushed and biased.

The power struggle, observers say, could be a very effective recruitment tool for IS, potentially helping it attract more Taliban turncoats.