REUTERS - Russia's Defense Ministry said on Wednesday that Russian airstrikes in Syria had killed one of ISIS' most prominent leaders, Abu Muhammad al-Adnani, on Tuesday.

The ministry said that Adnani was one of up to 40 rebels killed by the airstrikes in Syria's region of Maaratat-Umm Khaush, in the Aleppo province.

On Tuesday, a U.S. defense official told Reuters the United States targeted Adnani in a strike on a vehicle travelling in the Syrian town of al-Bab.

Adnani had been one of the last living senior members, along with self-appointed caliph Abu Bakr al-Baghdadi, that founded the group and stunned the Middle East by seizing huge tracts of Iraq and Syria in 2014.

As ISIS spokesman, he was its most visible member. As head of external operations, he was in charge of attacks overseas, including Europe, that have become an increasingly important tactic for the group as its core Iraqi and Syrian territory has been eroded by military losses.