More than 80 Bosnian children are in Islamic State-held territory in Syria and Iraq and represent a "time bomb" that could pose a major security risk when they return, a study said on Monday.

Bosnian Muslims are the largest group from the Western Balkans fighting for Islamic State in Syria and Iraq, alongside fighters from countries such as Kosovo, Albania, Macedonia, Montenegro and Serbia.

The study by the non-profit Sarajevo-based Atlantic Initiative, which made an advance copy available to Reuters, found that the number of adult male fighters, estimated at 188 in the three-year period to end-2015, had dropped to 91, after 47 returned to Bosnia and 50 had been killed.

As of April, less than half of Bosnians in Syria were men of military age, while there were also 52 women and 80 children. Some children, who went to the region with their families, have joined Islamic State combat units, the study said.

According to witnesses and social media, boys of 13 or 14 undergo military training before being sent to join fighting formations. At least one minor from Bosnia had been killed as a combatant, the study said, urging Bosnian authorities to prevent children from being taken to conflict zones.

"We are seeing a completely new generation of children who were raised on the battlefield or near the battlefield," said Vlado Azinovic, a co-author of the study. "They are like a time bomb for any country they may end up in."

Departures from Bosnia and returns from Syria had almost completely stopped by early 2016 because Bosnian authorities were prosecuting more aspiring fighters as well as those who returned, the study said.

Bosnia's Muslims are generally moderate but some have adopted radical Salafi Islam from foreign fighters who came to the country during its 1992-95 war to fight alongside Muslims against Orthodox Serbs and Catholic Croats.

Some of them have formed illegal communities which the moderate national Islamic organization wants to shut down. The study said Islamic community officials or property may become a target of possible retaliatory attacks.