The Islamic State group said Tuesday its spokesman Abu Mohamed al-Adnani was killed in the Syrian province of Aleppo, as the US confirmed it had targeted him in the same area.

Quoting an IS "military source," the group's news agency Amaq said: "Sheikh Abu Mohamed al-Adnani... was martyred while surveying operations to repel the military campaigns against Aleppo."

It said he died after a "long voyage crowned by sacrifice" and vowed "revenge" at the hands of a "new generation born unto the Islamic State."

In Washington, Pentagon spokesman Peter Cook said authorities were "still assessing the results of the strike."

Should Adnani's death be confirmed, it "would mark another significant blow to ISIL," he added in a statement.

Cook said the strike took place in Al-Bab, in the province of Aleppo.

Describing Adnani as the "principal architect" of the IS group's external operations and its main spokesman, Cook said he had "coordinated the movement of ISIL fighters, directly encouraged lone-wolf attacks on civilians and members of the military and actively recruited new ISIL members."

A US defense official called Adnani "one of ISIL's most senior leaders," stressing that he was "way more" than simply the group's spokesman.

The official said Adnani had played a major role in the group during some of the most high-profile attacks over the past year, including in Paris, at the Brussels and Istanbul airports, at a cafe in Bangladesh, as well as the downing of a Russian airliner in the Sinai and suicide bombings at a rally in Ankara.

The attacks have killed more than 1,800 people and wounded almost 4,000.

"Adnani was a legacy AQI (Al-Qaeda in Iraq) member, a Shura council member and the most publicly recognizable official in ISIL," the official said.

- 'Wider decline' in IS? -

In September 2014, the US government designated Adnani a "global terrorist" and the State Department has offered a $5 million reward for anyone who supplies information "that brings him to justice."

"The US military will continue to prioritize and relentlessly target ISIL leaders and external plotters in order to defend our homeland, our allies and our partners, while we continue to gather momentum in destroying ISIL's parent tumor in Iraq and Syria and combat its metastases around the world," Cook said.

Analysts have described Adnani as a key figure in the jihadist group.

"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.

Adnani was originally from the western Syrian province of Idlib and joined the jihadist movement in Iraq, where he served now slain Al-Qaeda leader Abu Musab al-Zarqawi and held several positions.

Aymenn Jawad Tamimi, an expert on jihadist groups, said Adnani's death was "significant symbolically and in pointing to the wider decline of the Islamic State."

"If a coalition air strike hit him, it shows intelligence penetration by the coalition is very high. Otherwise it would not have been possible to take out so many high-ranking figures," he said.

Another analyst, Charles Lister, tweeted that Adnani's death was a "big blow to IS."

Amaq did not say how Adnani was "martyred."

IS has regularly urged followers to target disbelievers.

Adnani made such a request in September 2014, calling on supporters to use stones, knives or even vehicles in their attacks to kill French or US citizens.

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