Two of the world's most powerful militaries are claiming responsibility for the airstrike that killed one of Islamic State's senior leaders, spokesperson and strategist Mohamed al-Adnani, in Syria this week.

On Wednesday, the Russian Defense Ministry said it killed Adnani and 40 other fighters with an airstrike carried out by an Su-34 bomber in the town of Maaratat-Umm Khaush in Aleppo province.

The Russian claim came after the Pentagon said on Monday that it targeted Adnani, but that it was still "assessing the results of the strike," which occurred in al-Bab, a different town in Aleppo province. The Pentagon has not confirmed Adnani's death.

A US defense official told CNN on Wednesday that the Pentagon doubts Russia's claim — but if he is dead, the US is responsible.

"It would be laughable but for the real humanitarian suffering Russia has inflicted," the official said. "We stand by the statement we made yesterday."

The Islamic State announced Adnani's death earlier on Monday through its Amaq news agency. The group said that Adnani was killed "while surveying the operations to repel the military campaigns against Aleppo."

Adnani was widely considered to be the militant group's second in command and the likely successor to current leader Abu Bakr al-Baghdadi.

He was also credited with overseeing the group's slickly-produced propaganda videos, which typically show torture, executions, and bombings.

Pentagon spokesperson Peter Cook said in a statement that Adnani's death would represent "another significant blow to ISIL."

"Adnani has served as principal architect of ISIL's external operations and as ISIL's chief spokesman," the statement said, using an alternate acronym for ISIS. "He has coordinated the movement of ISIL fighters, directly encouraged lone-wolf attacks on civilians and members of the military and actively recruited new ISIL members."

But Max Abrahms, a terrorism expert Northeastern University, said Adnani's death follows significant territorial losses by the group over the past few months.

"I think the Islamic State is really struggling, and this killing is a reflection of the group's downward trajectory, and will expedite the group's declining capabilities," he said.

He added that while the killing might be a blow to the Islamic State's morale, there's no established consensus about whether killing leaders is actually an effective way to defeat militant groups. He said that in some cases, the replacements have been even more radical than their predecessors, but this probably won't be the case with the Islamic State.

"It's hard to imagine that Adnani's successor will be more radical than Adnani," he said. "From top to bottom, Islamic State members are about as radical as they come. They want to attack every country, and kill as many people as possible."