Islamic State group spokesman and top strategist Abu Mohamed al-Adnani has been killed in Syria, the group said, with both Washington and Moscow claiming credit.
Adnani was IS's propaganda chief, top recruiter and the reported mastermind of a string of IS-claimed attacks in the West.
In Washington, the Pentagon said US-led coalition forces had targeted Adnani in an air strike in Syria's Aleppo province on Tuesday but did not immediately confirm his death.
Russia's military said one of its air strikes had killed Adnani in a bombing raid Tuesday that left up to 40 IS jihadists dead.
Regardless of who was responsible, analysts say his death will be a major blow to IS, which has suffered a series of setbacks this year including territorial losses in Syria and Iraq and the killings of other top figures.
Adnani was "the most viscerally aggressive ISIS leader in the public eye," said Charles Lister, a senior fellow at the Middle East Institute, using one of several names for IS.
"Without his explosive voice, ISIS may find it hard to inspire the intense levels of violence that it has managed to inspire of late."
Adnani, a Syrian born in 1977, was one of IS's most recognised leaders, at the heart of a sophisticated propaganda and recruitment machine that produced slick videos and sustained a huge social media presence.
He was reported to have been involved in organising a series of high-profile IS attacks abroad that killed hundreds, including in Paris, Brussels and Istanbul.
'Principal architect' of attacks
The IS-affiliated Amaq news agency announced Adnani's death late on Tuesday, saying he "was martyred while surveying operations to repel the military campaigns against Aleppo" in northern Syria.
Pentagon spokesman Peter Cook said US-led coalition forces had "conducted a precision strike" targeting Adnani near the IS-held town of Al-Bab in Syria's northern Aleppo province.
"We are still assessing the results of the strike but Adnani's removal from the battlefield would mark another significant blow to ISIL," he said.
Adnani "served as principal architect of ISIL's external operations and as ISIL's chief spokesman," Cook said.
The US-led coalition began air strikes against IS in mid-2014, a few weeks after the group seized control of large parts of Syria and Iraq.
Washington has vowed to "systematically eliminate" senior IS leaders and has put a $10 million bounty on the group's elusive leader Abu Bakr al-Baghdadi.
Both IS's second-in-command Abd ar-Rahman Mustafa al-Qaduli and its top military commander Omar al-Shishani have been killed in reported US strikes since March.
"Adnani's killing is a signal that IS can no longer protect its most senior leaders," said Baghdad-based expert on jihadists Hisham al-Hashimi.
He said it was clear that US intelligence had infiltrated top levels of IS and was increasingly aware of the movements of senior figures.
"I think the United States are very close to killing Baghdadi the next time," Hashimi said.
Russia started its own air war in Syria last September, backing its ally President Bashar al-Assad, and has repeatedly targeted IS positions.
The Russian defence ministry statement on Wednesday said an Su-34 warplane had struck a group of jihadists near the village of Um Hosh in Aleppo province.
"According to information confirmed through several intelligence channels, field commander Abu Mohamed al-Adnani was among those killed," the statement said.
It was the first time Moscow claimed to have killed a top-ranking IS leader.
Announced birth of 'caliphate'
Lister said the IS announcement of Adnani's death described him as Qurayshi -- referring to the tribe to which Islam's Prophet Mohammed belonged -- in an indication of his importance.
The use of the term "would seem to suggest ISIS may have been preparing him to be Baghdadi's successor as overall leader," Lister said.
"If that's indeed true, then Adnani's killing is arguably the most significant loss to ISIS since the late months of the US occupation of Iraq."
Adnani, from the western Syrian province of Idlib, joined the jihadist movement in Iraq where he served under the late local Al-Qaeda leader Abu Musab al-Zarqawi.
He was a founding member of IS, which evolved from Al-Qaeda in Iraq to become the preeminent global jihadist movement.
It was Adnani who in a June 2014 audio recording declared IS's establishment of a "caliphate" straddling Syria and Iraq and Baghdadi as "leader of Muslims everywhere".
"In the collective jihadist memory Abu Mohamed al-Adnani will always be the one who announced the 'restoration of the caliphate'," said expert Romain Caillet, describing the propaganda chief as "the most charismatic leader in IS".