The US-led coalition against the group will fight across the world to "ensure an enduring defeat of these terrorists and that any leaders who remain are delivered the justice that they deserve," a State Department spokesman said.

In Baghdadi's first purported appearance in five years, a cross-legged speaker said to be the world's most wanted man referred to the group's defeat last month in its final redoubt of Baghouz and threatened revenge attacks.

US government analysts "will review this recording and we will defer to the intelligence community to confirm its authenticity," the State Department spokesman said.

But regardless of the video's authenticity, the spokesman said that the Islamic State group, also known as ISIS, had been battered.

"ISIS's territorial defeat in Iraq and Syria was a crushing strategic and psychological blow as ISIS saw its so-called caliphate crumble, its leaders killed or flee the battlefield, and its savagery exposed," he said.