Top Islamic State group strategist Abu Mohamed al-Adnani, who was killed on Tuesday, played a major role in the group during some of the most high-profile attacks including at a cafe in Bangladesh, a US official said. 

The coalition forces had targeted Adnani, who had a $5 million bounty on his head, in an air strike in Syria's Aleppo province and that the Pentagon was still assessing the result.

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.

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.

A US defence official, who declined to be named, described Adnani as one of IS's most senior leaders and far more significant than simply the group's spokesman.

Adnani had played a key role during some of the most high-profile attacks over the past year, including in Paris, at the Brussels and Istanbul airports and at a cafe in Bangladesh, said the official.

"Most notably, he served as ISIL's chief of external operations, directing and inspiring major terrorist attacks outside of Iraq and Syria," the official added, using an alternative name for the group.

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.

"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.

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.

'ENCOURAGED LONE-WOLF ATTACKS'

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.

"He has coordinated the movement of ISIL fighters, directly encouraged lone-wolf attacks on civilians and members of the military and actively recruited new ISIL members."

Meanwhile, Russia's military yesterday said its air strikes had killed Adnani. In a statement, Russia's defence ministry said an Su-34 warplane on Tuesday killed "up to 40" IS fighters in a bombing raid near the village of Um Hosh in the province of Aleppo.

"According to information confirmed through several intelligence channels, field commander Abu Mohamed al-Adnani was among those killed," the statement said.

ANNOUNCED BIRTH OF 'CALIPHATE'

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 and in mid-2014 seized control of large parts of Syria and Iraq.

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' in June 2014," said expert Romain Caillet, describing the propaganda chief as "the most charismatic leader in IS."

A few months later Adnani released an audio recording calling for lone-wolf attacks on civilians in Western countries.

"If you cannot (detonate) a bomb or (fire) a bullet, arrange to meet alone with a French or an American infidel and bash his skull in with a rock, slaughter him with a knife, run him over with your car, throw him off a cliff, strangle him, or inject him with poison," he said.

Experts have warned of a possible increase in jihadist attacks in the West as IS faces growing pressure in Syria, where US officials estimate the group has lost 20 percent of the territory it once held, and in Iraq, where it has lost about 50 percent.