JAKARTA - More than 40,000 foreigners from about 110 countries had left home to join the Islamic State in Iraq and Syria (ISIS) since it declared its caliphate back in 2014.

Today, as ISIS loses its last swathes of territory in the Middle East to US-backed coalition forces, at least 5,600 of these foreigners have returned home, or fled to safe havens elsewhere.

These include citizens or residents of 33 countries, ranging from former Soviet republics to South-east Asian nations, such Indonesia and Malaysia.

According to a new report released on Tuesday (Oct 24) by New York-based security think-tank The Soufan Center (TSC), these returnees may pose security threats that governments the world over have yet to find adequate ways to address.

"While the numbers of foreign fighters and those believed to have wanted to become foreign fighters illustrate the scale of the problem, they do little to help address the next challenge: to uncover the identity of those who have left the so-called caliphate, find out where they have gone, assess what risk they pose, and take whatever action is possible to protect the public from harm," said the authors.

The report by TSC identified five categories of returnees, from those who failed to integrate within ISIS, to hardened terrorists sent to fight for the caliphate in other countries, as well as the domestic terror threat from supporters who never travelled to an ISIS front.

Women and children who joined ISIS but later returned home or were captured and are awaiting deportation are another subset of returnees who are even harder to quantify, assess and address.

"Without further research, it will be hard to judge the degree of their commitment to IS and their interest in becoming active rather than passive supporters," cited the authors, using another acronym for ISIS.

They added that while some female recruits may be "the meek and submissive wives" promised to young male ISIS fighters, most of them could also be seeking empowerment and the chance to break away from tradition and servitude.

"It is likely that at least some of the returnees will present a terrorist risk," said TSC. "Some women have also shown themselves to be successful recruiters, and as with men, female returnees may encourage others to commit terrorist crimes."

Counter-terrorism agencies in South-east Asia have already encountered several cases involving women who are non-returnees.

Philippine security forces on Oct 11 arrested Karen Aizha Hamidon, a 36-year-old Filipina suspected of recruiting, via social media, fighters from around the world for ISIS.

In Indonesia, at least three local women have been nabbed for plotting terror attacks in the past year.

On Aug 12, an Indonesian family of 18 comprising three men, six women, five teenagers including three girls, three children and an infant were repatriated from Syria. Indonesian police said they had pledged allegiance to ISIS and had been indoctrinated while there.

Children have also been similarly involved in the ISIS story, said TSC analysts, whose study indicated that among the 91 Malaysians in ISIS' ranks, were 12 women and 17 children, while of the 600 Indonesians who had left home to join ISIS, 113 were women.

Indonesian authorities recently recorded that 671 of its citizens had joined ISIS in the Middle East, among them more than a hundred children.

Jakarta-based radicalism expert Adhe Bhakti had also estimated that as many as 100 children may have made it to Iraq or Syria, but he too admitted it remains a challenge to determine the actual number.

ISIS propaganda videos and photos have frequently featured teenagers and children with weapons, including a chilling image of a toddler "beheading" a white teddy bear in front of the group's black flag which appeared last year.

One video posted then showed boys from Indonesia and Malaysia training with firearms at a paramilitary camp in Syria.

"The caliphate has regarded anyone over 15 as an adult, but children as young as nine have been trained to use weapons and taught to kill," said TSC. "From 2014 to 2016, IS is believed to have recruited and trained more than 2,000 boys between the ages of nine and 15 as 'Cubs of the caliphate'."

International Centre for Political Violence and Terrorism Research senior analyst Jasminder Singh said there has been chatter about the training of next-generation "mujahidins" in South-east Asia, even as pro-ISIS militants in the southern Philippine city of Marawi have been flushed out by troops.

"As a pro-ISIS posting warned, 'Marawi is just the beginning' and 'new cubs and soldiers' will be trained to fight the 'crusader forces'," he added.