At least 1,200 foreign fighters have returned to Europe, 900 to Turkey, 800 to Tunisia, and 760 to Saudi Arabia, according to data from the US-based research firm Global Strategy Group.
The group tracked "foreign fighters" that who have gone to Iraq and Syria and have been "stopped in, deported from, denied entry to watch-listed by Turkey, remain fighting, and returned."
Those who have been "stop-listed" in Turkey include more than 7,500 Saudi Arabians, 4,600 Tunisians, 4,000 Russians, 2,650 Tajikistanis, 2,800 Moroccans, 1,650 Azerbaijanis, 2,600 people from France, and 1,900 Kazakhstanis.
The study estimates more than 5,000 people from "Central Asia" fought in Iraq and Syria, while around 500 have returned.
In the report, 7 people have returned to the United States out of an estimated 129, and 60 of 180 to Canada.
"There are now at least 5,600 citizens or residents from 33 countries who have returned home. Added to the unknown numbers from other countries, this represents a huge challenge for security and law enforcement entities," the report found.
"States have not found a way to address the problem of returnees. Most are imprisoned, or disappear from view," the report adds.
As ISIS leader Abu Bakir al-Baghdadi's whereabouts remain unknown, some ISIS members may switch their allegiance to Ayman al-Zawahiri or Hamza, the son of Osama bin Laden, the report posits.
Officials in Iraq and the Kurdistan Region have previously cautioned that the military defeat of ISIS doesn't defeat the ideology.
"Its [ISIS] appeal will outlast its demise and while it will be hard to assess the specific threat posed by foreign fighter and returnees, they will present a challenge to many countries for years to come," the study reported.
