The group last week announced Abu Ibrahim al-Hashimi al-Quraishi as its new leader after a US commando raid killed Abu Bakr al-Baghdadi, the chief of the ultra-violent movement.

A senior US official, speaking on condition of anonymity, said that monitoring of Islamic State social media indicates that even followers did not know much about Hashimi.

"That has become a major issue in, if you will, the ISIS social media world. This guy appears to be a nobody," the official told reporters.

"What little we know about him, we're not impressed. And if he's in Iraq or Syria, we don't think he's too long for the world anyway," he said.

The official declined to offer further details, including whether, like Baghdadi, Hashimi had been detained at some point by US forces.

President Donald Trump last week tweeted of the group's new leader that "we know exactly who he is," but few details have emerged about Hashimi.

Baghdadi, who led the Islamic State group since 2014, had become the world's most wanted person for the militants' grisly attacks both in the region and abroad.

The Islamic State group confirmed that the 48-year-old Iraqi died in the raid in Syria's northwestern Idlib province.