The Pentagon confirmed Monday that a U.S. airstrike in Syria last month killed the Islamic State group's spokesman and chief strategist Abu Mohammad al-Adnani. The Pentagon statement contradicts Russia's claims late last month that one of its planes had killed Adnani.

Last week, the ISIS-run Amaq news agency said Adnani was "martyred while surveying the operations to repel the military campaigns in Aleppo," and vowed to avenge his death. The group also said that Adnani was killed in al-Bab in Syria's devastated Aleppo province on Aug. 30. Adnani, who was one of ISIS' most high-profile figures after leader Abu Bakr al-Baghdadi, had a $5 million bounty on him.

Pentagon Press Secretary Peter Cook said Monday that "the U.S. precision airstrike" specifically targeted Adnani and removed ISIS' "chief propagandist, recruiter and architect of external terrorist operations."

"It is one in a series of successful strikes against ISIL leaders, including those responsible for finances and military planning, that make it harder for the group to operate," Cook added, referring to the terrorist group with its acronym.

However, Moscow had claimed on Aug. 31 that its Russian Su-34 bomber in Aleppo killed Adnani along with nearly 40 other Syrian rebels.

Adnani is listed as the leader of ISIS' Emni group, a unit that orchestrates attacks outside of the Middle East, by the Counter Extremism Project. The U.S. State Department named him a terrorist in 2014 and later described him as "the main conduit for the dissemination of ISIL messages."

The U.S. said he had "coordinated the movement of Isil [ISIS] fighters, directly encouraged lone-wolf attacks on civilians and members of the military and actively recruited" new members.

Pentagon's Cook also said Monday that U.S. forces "will continue to target ISIL leaders, relentlessly pursue its external plotters, and, working with our partners on the ground, we will not rest in our efforts to destroy ISIL's parent tumor in Iraq and Syria, combat its metastases around the world, and protect our homeland."