Story highlights The strike took place on Friday near the Syrian town of al-Shaddadi

A defense official said 12 additional ISIS fighters were likely killed along with Shishani

Washington (CNN) A top ISIS leader, known as Omar "the Chechen" al-Shishani, was "critically injured" in a U.S. airstrike in northeastern Syria last week, activists for a Syrian opposition group said Wednesday.

The U.S. military and intelligence community was assessing Tuesday whether the strike had killed al-Shishani, several U.S. officials told CNN. The strike took place on Friday near the Syrian town of al-Shaddadi. There were also 12 additional ISIS fighters likely killed in the strike, Pentagon spokesman Peter Cook said in a statement.

Al-Shishani has a reputation as one of ISIS' most capable commanders. There has been a $5 million reward on his head from the U.S. State Department. Shishani is a former member of an elite Georgian military unit.

"Batirashvili is a battle-tested leader with experience who had led ISIL fighters in numerous engagements in Iraq and Syria," Cook said in the statement.

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