The Islamic State (IS) group says one of its most prominent and longest-serving leaders has been killed in what appeared to be an American air strike in Syria, depriving the militant group of the man in charge of directing attacks overseas.

US assessments often take days and often lag behind official announcements by militant groups on the ground.

Adnani was one of the last living senior members, along with self-appointed caliph Abu Bakr al-Baghdadi, who founded the group and stunned the Middle East by seizing huge tracts of Iraq and Syria in 2014.

As Islamic State’s spokesman, Adnani was its most visible member.

As head of external operations, he was in charge of attacks overseas, including Europe, that have become an increasingly important tactic for the group as its core Iraqi and Syrian territory has been eroded by military losses.

Advances by Iraq’s army and allied militia towards IS’s most important possession of Mosul have put the group under new pressure at a moment when a US-backed coalition has cut its Syrian holdings off from the Turkish border.

Those military setbacks have been accompanied by air strikes that have killed several of the group’s leaders, undermining its organisational ability and dampening its morale.

A US counter-terrorism official who monitors IS said Adnani’s death would hurt the militants “in the area that increasingly concerns us, as the group loses more and more of its caliphate and its financial base … and turns to mounting and inspiring more attacks in Europe, south-east Asia and elsewhere”.

Under Adnani’s auspices, the militant group launched large-scale attacks, bombings and shootings on civilians in countries outside its core area, including France, Belgium and Turkey.