The Islamic State group's battle-tested equivalent of a defense minister is believed to have been killed in a US air strike in northeastern Syria, a US official here said.

The target of the March 4 attack was Omar al-Shishani, a red-bearded Georgian fighting with the jihadist group in Syria, the Pentagon said Tuesday, cautioning that results of the operation were still being assessed.

A US official speaking on condition of anonymity later said Shishani "likely died" in the assault by waves of US warplanes and drones, along with 12 other IS fighters.

Al-Shishani is the nom de guerre of Tarkhan Batirashvili, who ranked among the most wanted under a US program with a $5 million bounty on his head.

The United States stopped short of declaring him dead.

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 described Shishani as "a battle-tested leader with experience who had led ISIL fighters in numerous engagements in Iraq and Syria."

His death, if confirmed, would hinder IS's foreign recruitment efforts, especially from Chechnya and the Caucasus regions, and its attempts to defend its strongholds in Syria and Iraq, according to the Pentagon.

Shishani's father, Taimouraz Batirashvili, told the Russian news agency Interfax that he was unable to confirm the death. "I know nothing about the death of my son. They announce his death almost every month."

The US Treasury designated Shishani a foreign terrorist fighter in 2014, and said he maintained "unique authority" within IS.

The Georgian was "the ISIL equivalent of the secretary of defense," the US official said, using an alternative acronym for the group.

In the recent assault, waves of US aircraft struck 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.

- Chechen rebel -

The US official said it was "unusual and noteworthy" that Shishani had traveled from IS's self-proclaimed capital of Raqa to Al-Shadadi.

"This was likely to bolster the sagging morale of ISIL fighters there, who have suffered a series of defeats by Syrian Democratic Forces," the official said, alluding to one of the local, US-allied fighting groups.

Shishani comes from a town in Georgia that is populated mainly by ethnic Chechens, the official said.

He fought as a Chechen rebel against Russian forces before joining the Georgian military in 2006, and fought Russian forces again in Georgia in 2008.

After being discharged from the Georgian military on health grounds, he entered Syria in 2012 and joined IS the next year.

Among his feats on his way to the top ranks of Islamic State military operations, Shishani turned one rebel group into an effective fighting force to take on the Syrian army by "mixing Syrians who knew the terrain with the Chechens' fighting ability," the US official said.

Shishani is believed to have led a prison in Tabqa near Raqa where foreign hostages may have been held.

He later headed IS military operations in northern Syria, according to the US official.

Many foreign IS fighters hail from the former Soviet republics -- in almost equal numbers as those from Western Europe -- according to the US-based intelligence consultancy the Soufan Group.