Islamic State spokesman Adnani 'killed in northern Syria'

The IS-run Aamaq news agency said on Tuesday that Adnani was "martyred while surveying the operations to repel the military campaigns in Aleppo," without providing further details.

His death, if confirmed, would be a severe blow to the group, which has been on the retreat in Syria and Iraq.

Adnani, who served as the voice of IS over the past few years, is also a senior commander and one of 43 original founders of the terrorist group.

Of the original founders, 41 have been killed so far according Iraqi terrorism expert Hisham Alhashimi.

Adnani has released numerous, lengthy audio sermons online in which he urged followers to carry out attacks.

Earlier this year, he called for massive attacks during the Muslim holy month of Ramadan.

He is also believed to be the head of the IS group's external operations, and is possibly responsible for recent terror attacks in France, Turkey and elsewhere.

He swore allegiance to Abu Musab al-Zarqawi, the leader of al-Qaeda in Iraq and rose through the jihadist ranks

Several terrorism experts have also indicated that Adnani was the presumptive heir to IS leader Abu Bakr al-Baghdadi.

There was no immediate comment or confirmation from Washington of his death.

Aamaq vowed to revenge against the "filthy cowards in the sect of disbelief." It said a generation raised in IS-held territory will take revenge.

Adnani, born Taha Subhi Falaha, was one of the first foreign fighters to cross the border from his native Syria to fight US troops after the 2003 invasion of Iraq.

He swore allegiance to Abu Musab al-Zarqawi, the leader of al-Qaeda in Iraq and rose through the jihadist ranks.

Adnani was reportedly captured in 2005 and imprisoned at a facility run by the US forces, but was freed in 2010.

His death comes less than six months after two other top IS officials – finance minister Haji Iman and third in command Abu Omar al-Shishani – were killed.

The Islamic State group has suffered a string of defeats in recent weeks, including in Syria's northern Aleppo province, where Turkish troops and allied Syrian rebels drove IS out of the border town of Jarabulus last week.

In Iraq, the group has lost its strongholds in Fallujah and Ramadi, in the western Anbar province.

It still controls Mosul, but Iraqi forces are gearing up for a long-awaited operation to retake the country's second largest city.