Senior Islamic State commander Omar al-Shishani has been “clinically dead” for several days following a US air strike in northern Syria, according to a monitoring group.

Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, said on Sunday: “Shishani is not able to breathe on his own and is using machines. He has been clinically dead for several days.”

Abdel Rahman said the notorious commander, known as Omar the Chechen, was in a hospital in the northern province of Raqqa, Isis’s bastion in Syria.

A US official said on 9 March that Shishani “likely died” in a barrage of US-led air strikes on 4 March in north-eastern Syria.

The official branded Shishani “the Isil equivalent of the secretary of defence”, using another acronym for the group.

Abdel Rahman at the time said the jihadist had been “seriously injured” in the strike on his convoy, but that he had not died.

Shishani was one of the Isis leaders most wanted by Washington, which put a $5m bounty on his head.

Shishani comes from the Pankisi Gorge, a mainly ethnic Chechen region of the former Soviet state of Georgia.

As early as May 2013, when Isis was just emerging in Syria, he was appointed the group’s military commander for the north of the country.

While Shishani’s exact rank is unclear, Richard Barrett of the US-based Soufan Group has described him as Isis’s “most senior military commander”, adding that he has been in charge of key battles.

Shishani is not, however, a member of Isis’s political leadership, a structure that is even murkier than its military command.

The lack of a US presence on the ground makes it difficult to assess the success of operations targeting militants in Syria, and Shishani’s death has been falsely reported several times.