Russia has been the No. 1 source country of foreign fighters for the Islamic State, according to a new report by a private security-intelligence firm, which also claims that thousands of the terror group’s fighters have returned to their home countries after losing territory in Syria and Iraq.

The report released Tuesday by the New York-based firm The Soufan Group said “there are now at least 5,600 citizens or residents from 33 countries who returned home” in a situation that “represents a huge challenge for security and law enforcement entities” around the world.

“As the so-called Islamic State (IS) loses territorial control of its caliphate, there is little doubt that the group or something similar will survive the worldwide campaign against it so long as the conditions that promoted its growth remain,” the report stated. “Its appeal will outlast its demise, and while it will be hard to assess the specific threat posed by foreign fighters and returnees, they will present a challenge to many countries for years to come.”

The Soufan Group, headed by former FBI Agent Ali Soufan, has made headlines with its analysis of ISIS foreign fighter trends in years past. A 2015 report by the group had listed the tiny North African nation of Tunisia as the No. 1 source country of the foreign fighters at that time.

Tuesday’s report updated the numbers, noting that Tunisia’s government has since dramatically revised — from 6,000 down to less than 3,000 — the number of Tunisians it believes have traveled to join ISIS.

With that as a backdrop, the latest report maintains that some 3,417 fighters traveled from Russia to join the terror group in Syria and Iraq, compared with 3,244 from Saudi Arabia, 3,000 from Jordan and 2,926 from Tunisia. France is also listed as a top source country, with some 1,910 having traveled from there to join IS.

Tunisia still ranks as the country that the highest number of foreign fighters have returned to in recent years, according to the Soufan report, which said some 800 fighters have returned to the nation.

Only 400 of the more than 3,400 Russian foreign fighters have returned home and just 271 of the more 1,900 French foreign fighters have returned home, the report said.

With regard to the United States, the report contends that about 129 U.S. foreign fighters have either gone to Syria and Iraq, been stopped in, deported from, denied entry to, or watch-listed by Turkey. Seven foreign fighters are listed as having returned to the United States.

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