By Phil Stewart and Warren Strobel

WASHINGTON (Reuters) - A red-bearded Islamic State commander described by American officials as the group's de facto minister of war may have been killed in an air strike in Syria on Friday by the U.S.-led coalition, several U.S. officials said on Tuesday.

Abu Omar al-Shishani, also known as Omar the Chechen, ranked among the most wanted militants under a U.S. reward program that offered up to $5 million for information to help remove him from the battlefield.

Born in 1986 in Georgia, which was then still part of the Soviet Union, Shishani had a reputation as a close military adviser to Islamic State's leader Abu Bakr al-Baghdadi, who was said by followers to have relied heavily on him.

He may have been killed during a coalition strike on March 4 near the town of al-Shadadi, which U.S.-backed forces from the Syrian Arab Coalition captured from the Islamic State last month. Still, the United States still appeared unwilling to declare Shishani dead.

Two U.S. officials expressed optimism about the strike but acknowledged that a determination about Shishani's fate was not certain and that the results of the operation still were being reviewed. A third official limited himself to saying Shishani was targeted in the strike.

A fourth official, speaking to Reuters on condition of anonymity, said the strike targeted a vehicle believed to be carrying Shishani, but declined to offer further details.

An official in the Syrian Kurdish YPG militia, which has been fighting Islamic State in the al-Shadadi area, said it had received information al-Shishani was killed but had no details and had been unable to confirm the death. The official declined to be identified because of the sensitivity of the matter.

The U.S. State Department described Shishani as a senior Islamic State commander and Shura Council member based in al-Raqqa, the Islamic State's de facto capital in Syria.

It said he was identified as the Islamic State's military commander in a video distributed by the group in 2014.

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Shishani, whose name was originally Tarkhan Tayumurazovich Batirashvili, oversaw a prison facility near Raqqa where Islamic State possibly held foreign hostages.

If confirmed, the strike would be one of the most successful operations to take out Islamic State's leadership in Iraq and Syria since May, when U.S. special operations forces killed the man who directed the group's oil, gas and financial operations.

In November, a U.S. air strike killed Islamic State's senior leader in Libya, known as Abu Nabil.

(Additional reporting by Jonathan Landay; Editing by Bill Trott)