The Britain-based Syrian Observatory for Human Rights said that according to its sources the March 4 strike had indeed targeted the jihadist's convoy, killing his bodyguards, while he himself "was seriously injured".
"He's not dead," the Observatory's director Rami Abdel Rahman told AFP.
"He was taken from the province of Hasake to a hospital in Raqa province where he was treated by a jihadist doctor of European origin," he said. Raqa is IS's main stronghold.
The United States has stopped short of declaring Shishani dead, but a US official speaking on condition of anonymity said Shishani "likely died" in the assault by waves of US warplanes and drones, along with 12 other IS fighters.
The US official branded Shishani "the ISIL equivalent of the secretary of defence," using another acronym for the group.
Shishani was one of the IS leaders most wanted by Washington which put a $5 million bounty on his head.
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.
Pentagon spokesman Peter Cook on Tuesday described Shishani as "a battle-tested leader with experience who had led ISIL (IS) fighters in numerous engagements in Iraq and Syria."
The March 4 strikes took place near Al-Shadadi, a town in northeastern Syria that was retaken from IS last month by local anti-IS fighters allied with the US-led coalition.