The Russian defense ministry had earlier said an Su-34 warplane killed "up to 40" IS fighters -- including Adnani -- in a bombing raid near the village of Um Hosh in the province of Aleppo on Tuesday.
"That's a joke," a US defense official told AFP when asked about Moscow's Adnani claim, speaking on condition of anonymity to discuss military operations.
"It would be funny if not for the character of the campaign the Russians have undertaken in Syria."
Russia has been flying its own bombing campaign in Syria in support of leader Bashar al-Assad since last year, but this is the first time they have claimed to have killed a top-ranking IS leader.
The United States says coalition forces had targeted Adnani, who had a $5 million bounty on his head, in an air strike in Aleppo province on Tuesday, but the Pentagon is still assessing the result.