The White House said Wednesday it had no evidence that Russia successfully killed the Islamic State's spokesman and top recruiter Mohammad Adnani.

"There have been some reports of Russian officials indicating that Russia may have been involved in this effort," Earnest said, Air Force One, unprompted by reporters. "I am not aware of any facts that would substantiate a claim like that."

Russia's Ministry of Defense said on its official Facebook page that it had killed the terror leader, and claimed that Adnani was one of up to 40 ISIS fighters killed by a Russian airstrike in Aleppo province.

But a U.S. defense official dismissed the claim as preposterous, and told CNN, "It would be laughable but for the very real humanitarian suffering Russia has inflicted." We stand by the statement we made yesterday. We conducted a strike that targeted al-Adnani. We are assessing the results of that strike."

On Tuesday, in a rare public admission, the Islamic State announced Adnani's death in statement from the group's Amaq news agency, although it did not reveal the cause of death and vowed to seek revenge.

If the U.S. confirms his death, Earnest said it would constitute a "significant blow" to the terrorist group.

"He is often described as a spokesperson for the group, but he also has played an active role in recruiting supporters for ISIL," Earnest said. "He has also been a leading proponent of a strategy that encourages individuals to carry out lone wolf attacks around the world."