Omar al-Shishani. Thomson Reuters

The US believes that it successfully killed one of ISIS' most successful military leaders in a March 4 airstrike in Syria.

The attack in northeastern Syria was aimed against ISIS' "minister of war," Omar al-Shishani, aka Omar the Chechen. It was carried out with multiple waves of manned and unmanned aircraft. The strike flattened an area the US believes was holding Shishani.

His death would most likely function as a major setback for ISIS. Aside from the group's leader, Abu Bakr al-Baghdadi, the Georgian ex-commando Shishani has been the most recognizable and popular member of the powerful terrorist group.

US now believes #ISIS military commander "Omar the Chechen" died from injuries sustained in March 4th US airstrike in northern #Syria. — Jon Williams (@WilliamsJon) March 14, 2016

And Shishani's status, combined with his ethnicity, helped draw numerous foreign fighters from the Caucasus region into Syria to fight alongside ISIS. His death would therefore also function as a major moral loss.

Not everyone agrees with the US's assessment that the airstrikes managed to kill Shishani. A monitoring group, the Syrian Observatory for Human Rights, reports that the airstrike did not kill Shishani but instead left him severely injured and "clinically dead."

"Shishani is not able to breathe on his own and is using machines," Rami Abdel Rahman, head of the monitoring group, told the AFP. "He has been clinically dead for several days."

Even if this were the case, it would still be a blow to ISIS. Though Shishani did not hold a political role within the group, he had managed to carry out some of its most successful military operations. It was Shishani who posed with the stolen US Humvees that ISIS had seized from Mosul, Iraq, and brought back into Syria.

And it was Shishani who led successful ISIS military campaigns throughout Syria as well as a blitz through western Iraq that put the group within 100 miles of Baghdad.