More than 15,000 citizens from 80 countries have joined the Islamic State in Iraq and Syria, estimates from U.S. intelligence indicate. Of these, one particular group has become especially prominent.

Among high-ranking leaders targeted this week by the Obama administration was Tarkhan Tayumurazovich Batirashvili, a top Islamic State leader who has appeared in many of its social media posts and videos. Within the movement, he goes by the name Omar al-Shishani, which translates to “Omar the Chechen.”

He’s  just one of more than 800 Russian militants who have joined the Islamic State. Experts say Batirashvili and other Chechens are emerging as one of the largest groups of foreign fighters in Syria, which could pose a serious problem for U.S. efforts to fight the Islamic State, as the Voice of America reported Saturday.

“The tactical prowess of the fighters from the Caucasus, who have battled Russian troops for three decades, is evident on the Syrian battlefield,” wrote Bill Roggio for the Long War Journal.

Batirashvili was one of the 11 individuals sanctioned by the U.S. Treasury Department Wednesday. The U.S. State Department also added Murad Margoshvili, a well-known Chechen leader who reportedly built a terrorist training base in Syria to teach foreign fighters combat skills,  and the group Jaish al-Muhajireen wal-Ansar (JAMWA) to its list of foreign terrorist organizations.

“In the jihadist world, there are levels of fear and respect, and the Chechens occupy the top,” Patrick Skinner, director of special projects for security consultancy firm Soufan Group, told NBC News in July, adding that the Pankisi Gorge in Georgia is known as “the Harvard of terrorist upbringing.”