A senior Russian official confirmed that hundreds of Russian mercenaries are thought to be fighting in Syria in an interview with Interfax on Friday.
"Recruitment of mercenaries really does happen. We think that about 300 to 400 people from the territory of our country went [to Syria]. They will return, and, naturally, this poses a major danger," Sergei Smirnov, first deputy director of the Federal Security Service, said.
The statement confirmed concerns expressed by President Vladimir Putin in his recent op-ed for The New York Times, in which he voiced "deep concern" over the "hundreds of militants from Western countries and even Russia" currently fighting in the war-torn country.
"Might they not return to our countries with experience acquired in Syria?" Putin asked.
The Russian legal system has no precise guidelines for stopping mercenary activities, Smirnov told reporters while attending a Yaroslavl meeting of the Regional Anti-Terrorist Structure, or RATS, a division of the Shanghai Cooperation Organization.
Although there was no formal discussion concerning mercenaries at the RATS meeting, Smirnov said the different security officials present at the meeting acknowledged that the issue is one that their countries share.
The rebels fighting Syrian President Bashar Assad's regime are thought to come from a number of countries in the region. Earlier this year, USA Today reported that Saudi Arabia offered more than 1,200 death row inmates the choice between execution and being sent to join the fight in Syria.