(CNN) Russia said one of its airstrikes in Syria killed ISIS spokesman Mohammad al-Adnani -- a claim that a US official called preposterous.

On Wednesday, Russia's Ministry of Defense announced it killed the terror leader on the ministry's official Facebook page. It said Adnani was one of up to 40 ISIS militants killed by a Russian bomber in Aleppo province.

But a US defense official slammed Russia's assertion.

"It would be laughable but for the very real humanitarian suffering Russia has inflicted," the defense official said Wednesday. "We stand by the statement we made yesterday. We conducted a strike that targeted al-Adnani. We are assessing the results of that strike."

That attack was carried out by a US drone, two US defense officials told CNN on Wednesday.

On Tuesday, ISIS announced Adnani's death in a rare public statement. His death marks the highest-profile killing yet of an ISIS member.

A key deputy to ISIS leader Abu Bakr al-Baghdadi, he was floated to be his successor should anything happen to Baghdadi.

A statement from ISIS' Amaq news agency on Tuesday said Adnani died while inspecting military operations in the area of Aleppo. ISIS has not revealed his cause of death and said it was "determined to seek revenge" for the killing.

"After a journey filled with sacrifice and fight against non-believers, the Syrian Gallant knight, Abu Mohammed al-Adnani, joined the convoy of martyr leaders," ISIS said.

"To the filthy and coward nonbelievers and to the holders of the Christ emblem, we bring the good news, which will keep them awake, that a new generation in the Islamic State ... that loves death more than life ... this generation will only grow steadfast on the path to Jihad, stay determined to seek revenge and be violent toward them."

'Significant blow'

Pentagon press secretary Peter Cook said Tuesday that while the results of the US-led coalition's airstrike against Adnani were not clear, his "removal from the battlefield would mark another significant blow to ISIS."

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"Al-Adnani has served as principal architect of ISIS' external operations and as ISIS' chief spokesman. He has coordinated the movement of ISIS fighters, directly encouraged lone-wolf attacks on civilians and members of the military and actively recruited new ISIS members. The U.S. military will continue to prioritize and relentlessly target ISIS 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."

More than a spokesman

As spokesman, Adnani was the group's most prominent face, the first to announce the ISIS caliphate even before Baghdadi did.

But he was much more than a spokesman, CNN terrorism analyst Paul Cruickshank said.

Adnani was well-known for ordering operatives to attack countries participating in the coalition against ISIS. Western intelligence believes he had command responsibility for the November 2015 Paris attacks

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He is believed to have carried out those orders as one of the ISIS leaders overseeing its security service, known as Amniyat, which includes the external operations unit tasked with international attacks on the West. As such, the external operations unit reported up to Adnani.

A number of ISIS followers and members captured and questioned in the last two years, including French jihadists Faiz Bouchrane and Reda Hame, have attested to Adnani as head of the Amniyat.

"In some ways he's a more dangerous figure than Abu Bakr al-Baghdadi, because he's believed to be overseeing the external operations division of ISIS, and that's the part of ISIS which threatens the West, which carried out the attacks in Paris, which could one day carry out an attack in the US on a significant scale," Cruickshank said.

First to declare ISIS caliphate

Adnani has been with ISIS since its beginnings and was close to Baghdadi, CNN's Nic Robertson said. In June 2014, he was the first to declare a caliphate for parts of Syria and Iraq, indicating ISIS' aim to become a governing entity and not just a terrorist group.

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Born in Syria in 1977, Adnani was the most senior Syrian in ISIS, a trait that some analysts say may have counted against him in a leadership bid among the Iraqi-led terror group.

The US State Department officially labeled Adnani a terrorist in August 2014 and put a $5 million bounty on his head. It described him as "the official spokesman for and a senior leader" of ISIS, a position he obtained after becoming one of the first foreign fighters to oppose US-led forces in Iraq.

He was arrested in May 2005 in Anbar province and is believed to have spent some time between 2005 and 2010 at the US detention facility Camp Bucca.

After his arrival in Syria, he was appointed deputy of Abu Mohammed al-Jawlani, leader of the Islamic State of Iraq for the northern province, then as the security leader before assuming a key role in external operations for ISIS, according to the UN Security Council

'Crush their heads with rocks'

"He was the mouthpiece of ISIS. He said things like, 'If you can't shoot them, then stab them, and if you can't stab them, then crush their heads with rocks. If you can't do that, then drive your cars, your vehicles, to kill them," Robertson said.

"He absolutely tried to maximize every opportunity to instill fear in Syria and Iraq and the international community and send fighters overseas to attack."

Adnani said ISIS supporters in the West had a religious duty to launch lone-wolf attacks, a move analysts call a game changer that may have inspired attacks in North America, Europe and Australia.