A top leader of the Islamic State group, who was wounded in a U.S. airstrike last week, has died, the Obama administration confirmed Monday. Omar al-Shishani, a Chechen fighter believed to be one of ISIS’ most skilled commanders, was targeted near the Syrian town of al-Shaddadi.

Earlier reports suggested that Shishani, who held numerous senior military positions within the group, including "minister of war," had been badly wounded after airstrikes hit his convoy. Twelve others were also killed in a wave of strikes by drones and manned aircraft on March 4.

"We believe he subsequently died of his injuries," Pentagon spokesman Capt. Jeff Davis told the Agence France-Presse (AFP).

The Syrian Observatory for Human Rights said last week that Shishani had been taken to a hospital in Raqqa province where he was treated by "a jihadist doctor of European origin.” Later on Sunday, the monitoring group said that the ISIS leader had been "clinically dead" for several days.

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

U.S. officials suspect that Shishani was sent to the Shaddadi area to reinforce ISIS fighters following a series of defeats by the military, BBC reported. Last year, the U.S. reportedly offered a bounty of $5 million for Shishani.

Shishani spent years as a Chechen rebel fighting against the Russian military, and also gained experience fighting in the Georgian military in 2008 during the country’s brief war against Russia. Shishani joined rebel groups fighting in Syria in 2012, around the start of the Syrian civil war.  He pledged allegiance to ISIS in 2013, a U.S. official said, according to CNN, adding that he later was in charge of a prison near Raqqa where the Sunni-militant group may have held foreign hostages.