Moscow claimed credit Wednesday for killing ISIS' spokesman and chief strategist in an airstrike.
Abu Mohammed al-Adnani was one of up to 40 rebels who died Tuesday in attacks carried out by a Su-34 bomber in Maaratat-Umm Khaush, Syria, according to Russia's Defense Ministry. It cited "several intelligence channels."
A U.S. defense official dismissed that assertion. "We have no reason to believe Russia's claim," the official told NBC News. Earlier, a U.S. defense official told Reuters, "Russia's claim is a joke."
On Tuesday, the Pentagon said it could not yet confirm al-Adnani's death but announced that coalition forces had carried out a "precision strike" targeting him near Al-Bab, a Syrian town near the Turkish border.
The ISIS affiliated Amaq news agency reported on Tuesday that al-Adnani was killed "while surveying the operations to repel the military campaigns against Aleppo."
His death would represent a major blow to the extremist group, which has been on the retreat in Syria and Iraq.
Al-Adnani, whose real name is Taha Sobhi Falaha, persistently called for attacks against the West.
Born in Syria's northern province of Idlib, he is believed to have been aged in his late 30s. He crossed the border and joined al-Qaeda in Iraq, a precursor to ISIS, after the 2003 U.S.-led invasion.
A powerful orator, he went on to become the voice of ISIS. He released numerous audio files online in which he delivered fiery sermons urging followers to kill civilians in nations that supported the U.S.-led coalition against the group.
He was considered ISIS' second-in-command.