The IS group may be on the defensive in Syria and Iraq, but it now has thousands of foreign volunteer fighters who, once home again, will pose a major threat, experts warn.

Western authorities estimate some 25,000 to 30,000 fighters drawn by the call to jihad have thronged to the IS group's self-proclaimed "caliphate" in recent years.

While some have died and others continue to wage war, a substantial number are returning to their home countries as IS loses ground under an onslaught by the US-led international coalition.

"The flow of foreign fighters from western countries has fallen from 2,000 to about nothing a month," Albert Ford of the New America think tank told AFP.

"But that's only half the issue: What do you do about the 25,000 or 30,000 people that are in Syria or have been there that now want to go back? It's an issue that's not going to go away," he said.

In the 1980s, Arab volunteers flooded into Afghanistan to fight the Soviet Union in approximately equal numbers. After thwarting the Soviets, these so-called "Afghan Arabs" became the vanguard of several jihadist movements, while others carried out attacks across multiple countries.

"Once mobilized, a wave of foreign fighters is often difficult to demobilize," said a report by 20 US experts entitled "The Jihadi Threat -- ISIS, Al-Qaeda, and Beyond," which was published Monday.

"And foreign fighters who do demobilize are likely to remain an important part of the fabric of modern jihads, becoming facilitators or supporters who push the agenda forward, even if they do not join the fight itself," the report said.

In Europe, interviews carried out by special services and journalists after jihadists return from Syria and Iraq indicate that while the fighters may renounce violence, many still maintain the strong religious convictions that led them to join the movement.

- 'Pass through the net' -

"Total suppression of IS on the ground has nothing to do with what will happen in Western countries," Marc Sageman, a psychiatrist and former CIA agent in Pakistan during the Soviet invasion of Afghanistan, told AFP, speaking in French.

"In all countries, with the exception of perhaps the Netherlands and Denmark, it's time for rigor with the returning fighters. Politicians cannot afford to let even one person fall between the cracks and take action."

The hundreds and soon thousands of veterans of the jihadist movement in Syria and Iraq who are beginning to return pose a difficult problem, Katherine Zimmerman of the American Enterprise Institute said.

"Law enforcement is overwhelmed today, and it's going be even more so in a year in terms of the challenge that they face," she said.

"There will be more people, and they're going to be better networked. And there is only so much that you can do to stop them," Zimmerman added.

History has proven that it is necessary to closely monitor older jihadists who seem to have settled down, she said.

"Look at Cherif Kouachi, one of the Charlie Hebdo shooters. He was in prison in the mid-2000s. It took him years to activate," Zimmerman said.

For Nicholas Heras of the Center for a New American Security, the task at hand is not simple.

"It's extremely difficult to differentiate those who come back because they don't believe in the cause any more from those who come back to wage jihad in another form," he said.

"We need to engage with the families, with the communities. We need their help. We can't monitor these people 24/7," Heras said.