Up to 20,000 citizens from some 100 countries are believed to be fighting for ISIS in Iraq. Reuters

Nearly 1,700 Russian nationals are believed to be fighting alongside militants of the Islamic State group in Iraq, according to Alexander Bortnikov, director of the Russian Federal Security Service (FSB). Bortnikov reportedly said, on Thursday, that the number of Russians who are fighting for ISIS in Iraq has doubled over the last year.

“At the moment, some 100 countries are involved in the Iraqi conflict. The number of [foreign] participants that are fighting alongside Islamic State has grown from 13,000 to 20,000 -- that is our estimate,” Bortnikov said, expressing concerns over the rise in the number of foreign ISIS fighters, according to a report by RIA Novosti.

Bortnikov, who was in Washington, D.C., to participate in a counterterrorism summit, said that there is a possibility that Moscow and Washington might share intelligence to fight and defeat ISIS. He also added that participants at the summit had agreed not to politicize the issue of cooperation in efforts to tackle terrorism, RIA Novosti reported.

Besides people from Russia’s North Caucasus, militant groups are also attracting so-called "neophytes," or recent converts, Andrey Przhezdomsky, a spokesman for Russia’s National Antiterrorism Committee, had said in December, adding that “they are all currently either on wanted lists or subjected to investigations.”

The Associated Press (AP) reported last week that the U.S.-led coalition's airstrikes against ISIS in Syria and Iraq had failed to halt the influx of foreign fighters seeking to join ISIS. The report said that about 20,000 fighters from around the world, including 3,400 from Western countries, are estimated to have joined militant groups in Iraq and Syria.

“By now we know well that the impacts of violent extremism are by no means confined to one part of the world,” U.S. Secretary of State John Kerry said, according to a statement released Wednesday. “And the foreign fighter phenomenon is not limited to Syria and Iraq, though clearly we’ve seen the deadly impact of the flow of foreign fighters over the past weeks and months.”