Russia's claim that it carried out the airstrike in Syria that killed Islamic State's spokesman and chief strategist is "garbage," a U.S. official tells VOA.

In a statement, Russia's defense ministry said Russian warplanes killed up to 40 IS militants, including Abu Muhammad al-Adnani, during a bombing raid near Aleppo province on Tuesday.

A senior defense official told VOA that Russia's claim is "yet another attempt at disinformation."

The official said al-Adnani was being watched "for some time" and he was killed in an airstrike on a vehicle near Washiya, a village north of Aleppo.

Islamic State itself announced Abu Muhammad al-Adnani's death Tuesday, saying only that he was killed "while surveying the operations to repel the military campaigns against Aleppo."

A U.S. defense official said Adnani was directly involved in recruiting foreign fighters and also directed Islamic State's major attacks outside of its strongholds in Syria and Iraq.

"Significant operations carried out on his watch include the Paris attacks, the Brussels airport attack, the Istanbul airport attack, the downing of the Russian airliner in the Sinai, the suicide bombings during a rally in Ankara, and the attack on a cafe in Bangladesh," the official said. "In total, these attacks killed over 1,800 people and wounded nearly 4,000."

The Pentagon is still assessing the airstrike near Al Bab to confirm Adnani's death.

A government official in Washington told VOA that the U.S. been tracking several "high value" IS members in Aleppo province.

Adnani was reported to have been seriously injured eight months ago in Iraq, during fighting near the city of Haditha. He was born in Syria about 39 years ago and was a prominent member of the al-Qaida terror network before aligning himself with Islamic State, where he was considered second in rank to the group's leader, Abu Bakr al-Baghdadi.

The New York Times reported earlier this month that Adnani headed a special unit within Islamic State known as the Emni which organized and carried out attacks beyond the territory Islamic State held in Iraq and Syria.

"He oversaw the group’s external operations division, responsible for recruiting operatives around the world and instigating or organizing them to carry out attacks that have included Paris, Brussels and Dhaka, Bangladesh," the newspaper reported.

Adnani narrated an infamous statement from Islamic State nearly two years ago, calling on Muslims living in the West to strike out wherever and however they could.

“If you can kill a disbelieving American or European — especially the spiteful and filthy French — or an Australian, or a Canadian, or any other disbeliever from the disbelievers waging war, including the citizens of the countries that entered into a coalition against the Islamic State, then rely upon Allah, and kill him in any manner or way however it may be,” Adnani said in the recording.

The United Nations, which had Adnani on its list of suspected terrorists subject to financial sanctions, has described him as the leader of Islamic State in Syria and chief of its external operations.

The U.S. State Department had also offered a $5 million reward for information leading to Adnani's capture.