Abu Bakr al-Baghdadi, the self-proclaimed "caliph" terrorising Iraq and Syria, is a preacher who rose from obscurity to lead the world's most feared jihadist organisation.

His Islamic State group on Thursday released an audio recording purporting to be of Baghdadi, days after rumours that air strikes may have killed or wounded him.

Like much about Baghdadi, little is known about the strikes or their results, or even where they took place -- the US announced it had targeted IS leaders in north Iraq, but reports also emerged of a strike in the west.

With both areas outside government hands, verifying what transpired in either will be difficult if not impossible.

In the recording, the man said to be Baghdadi was defiant, vowing that IS's "march will not stop and it will continue to expand," and that his enemies would be drawn into a ground war.

Baghdadi has revived the fortunes of Iraq's struggling Al-Qaeda affiliate, turning it into the independent IS group, arguably the most brutal, powerful and wealthiest jihadist organisation in the world.

Under his leadership, IS spearheaded a militant offensive that overran much of Iraq's Sunni Arab heartland since June after seizing major territory in neighbouring Syria, and carried out a series of atrocities in both countries.

It launched a renewed drive in Iraq's north in August, pushing Kurdish troops back towards their regional capital Arbil and sparking a US-led campaign of air strikes and the deployment of up to 3,100 American soldiers in the country to advise and train its forces.

The group has killed hundreds of Iraqi and Syrian tribesmen who opposed it, attacked members of the Yazidi religious minority, sold women as slaves, executed scores of Iraqi security personnel and beheaded western journalists and aid workers on camera.

Baghdadi was declared a "caliph" on June 29 in an attempt to revive a system of rule that ended nearly 100 years ago with the fall of the Ottoman Empire, and ordered Muslims to obey him in a video from the northern city of Mosul.

The man now touted as the world's most prominent jihadist had rarely been seen in public before and his appearance in the video appeared to mark his growing confidence, but he has remained out of sight since the air strikes began.

- $10 million bounty -

Baghdadi, born in Samarra in 1971 according to Washington, apparently joined the insurgency that erupted shortly after the 2003 US-led invasion of Iraq, at one point spending time in an American military prison in the country.

In October 2005, American forces said they believed they had killed "Abu Dua", one of Baghdadi's known aliases, in a strike on the Iraq-Syria border.

But that appears to have been incorrect, as he took the reins of what was then known as the Islamic State of Iraq (ISI) in May 2010 after two of its chiefs were killed in a US-Iraqi raid.

Since then, details about him have slowly trickled out.

In October 2011, the US Treasury designated him as a "terrorist", and there is now a $10-million (7.3-million-euro) bounty for his capture.

This year, Iraq released a picture it said was of Baghdadi, the first from an official source, depicting a balding, bearded man in a suit and tie.

He is touted within IS as a battlefield commander and tactician, a crucial distinction compared with Al-Qaeda chief Ayman al-Zawahiri, and has attracted legions of foreign fighters, with estimates in the thousands, as well as supporters from around the world who distribute the group's propaganda online.

At the time Baghdadi took over ISI in April 2010, it appeared to be on the ropes after the "surge" of US forces combined with the shifting allegiances of Sunni tribesmen to deal it a blow.

But the group bounced back, expanding into Syria in 2013.

Baghdadi sought to merge with Al-Qaeda's Syrian franchise, Al-Nusra Front, which rejected the deal, and the two groups have mostly operated separately since.

But IS received a major boost this week from Egypt when the country's deadliest militant group, Ansar Beit al-Maqdis, pledged allegiance to Baghdadi.

The announcement is the most significant pledge of support for IS in the region outside Iraq and Syria, suggesting its influence over militant groups is overshadowing its once dominant Al-Qaeda rivals.