Abu Bakr al-Baghdadi rose from obscurity to lead the world's most infamous and feared jihadist group, but shuns the spotlight and cultivates an aura of mystery.

A recording purportedly of the Islamic State leader's voice was released on Thursday, the first such message in 2016.

It came as thousands of Iraqi forces started to mass around the city of Mosul, where Baghdadi publicly announced the creation of his "caliphate", the most ambitious and brutal experiment in modern jihad.

The recording released by the IS-affiliated Al-Furqan media group was a rare sign of life from Baghdadi, who is known as Caliph Ibrahim by his followers and has a $10 million US bounty on his head.

Air strikes by the US-led coalition supporting the anti-IS war in Iraq and Syria have depleted the leadership of the jihadist organisation in recent months.

Rumours have abounded about Baghdadi's own health and movements but his whereabouts remain unclear.

Shortly after jihadist fighters swept across swathes of Iraq in June 2014, Baghdadi appeared at the Great mosque of Al-Nuri to proclaim a "state" straddling Syria and Iraq in front of thousands of Muslim faithful.

The video of the appearance in Mosul -- which showed a man with a black and grey beard wearing a black robe and matching turban -- is the only one IS has released of Baghdadi to date.

- Low key -

"It is rather remarkable that the leader of the most image-conscious terrorist group is so low-key in terms of his own publicity," said Patrick Skinner, an analyst with the Soufan Group intelligence consultancy.

Baghdadi has been reported wounded in air strikes multiple times, but the claims have never been verified, and his apparent survival has added to his mystique.

"He has an element of mystery about him and seems to have achieved a lot more in real terms" than "old guard" jihadists such as Al-Qaeda chief Ayman al-Zawahiri, said Aymenn al-Tamimi, a fellow at the Middle East Forum.

Baghdadi revived the fortunes of Iraq's struggling Al-Qaeda affiliate, the Islamic State of Iraq (ISI), turning it into the independent IS group.

ISI was on the ropes when Baghdadi took over in 2010, but the group has bounced back, expanding into Syria in 2013 and then launching its sweeping offensive in Iraq last year.

Baghdadi "and his circle made a very, very strategic plan and then they went out and executed that plan; that inspires loyalty and confidence," said Skinner.

Until the first half of 2015, Baghdadi was the undisputed leader of the jihadist world but the steady decline of the "caliphate" will offer his rivals a chance to challenge his supremacy again.

- 'Family man' -

According to an official Iraqi government document, Baghdadi was born in Samarra in 1971 and has four children with his first wife -- two boys and two girls born between 2000 and 2008.

Another ex-wife, Saja al-Dulaimi, who was detained in Lebanon in 2014, described Baghdadi in a March interview to Swedish daily Expressen as "a normal family man" and university professor adored by his offspring.

Their seven-year-old daughter, Hagar, said she wanted to go to Europe to study.

An Iraqi intelligence report indicates that Baghdadi has a PhD in Islamic studies and was a professor at Tikrit University.

Baghdadi apparently joined the insurgency that erupted after the 2003 US-led invasion of Iraq, at one point spending time in an American military prison in the country's south.

Baghdadi's path to jihadist leader differed sharply from that of Osama bin Laden, who had huge wealth to rely on to help build Al-Qaeda, appeared in far more videos and was internationally known long before the September 11 attacks.

"His rise to fame really doesn't compare to other more publicised terror leaders. (Bin Laden) was famous by his name, and he made a show of his piety and low-key demeanour," Skinner said.

"Baghdadi seriously worked behind the scenes and then exploded into publicity when he was announced as the leader, but even then he didn't do publicity stunts," he said.

"He avoids the spotlight, and when he releases a speech, it is about the caliphate and its enemies, not himself."