The US now believes that it killed ISIS's military mastermind in an airstrike

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

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

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

The death of Shishani will likely function as a major setback for ISIS. Aside from ISIS's "caliph," Abu Bakr al-Baghdadi, the Georgian ex-commando Omar al-Shishani was the most recognizable and popular of the powerful terrorist group's leaders.

And Shishani's status, combined with his ethnicity, helped to draw a number of foreign fighters from the Caucasus region into Syria to help fight alongside ISIS. As such, his death will also function as a major moral loss for the militant group.

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

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

Even if this was the case, it would still be a blow to ISIS. Although Shishani did not hold a political role within the militant group, he had managed to carry out some of the most successful military operations for the militants. It was Shishani who posed with the stolen US Humvees that ISIS had seized from Mosul 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.