From mid-2014 to the end of March 2015 recruits surged by 71 per cent, according to a report submitted to the UN Security Council at the end of last month.
While the current flow of foreign fighters is at a historic high, Marc Pierini, a visiting scholar at Carnegie Europe and a former EU ambassador, said shifting realities on the ground means recruits are constantly coming and going. Consequently, hard figures are hard to confirm.
Still, the flow of foreign fighters has sharply increased from just several thousand a decade ago.
The report found that the majority of people are landing in Iraq and Syria, calling the region a “finishing school for extremists”.
The remaining 5,000 people have travelled to Afghanistan, Yemen, Pakistan and Somalia.
The report also warns that if the Islamic State armed group is defeated, it could result in these hardened fighters being inadvertently dispersed across the world.
Meanwhile, governments are scrambling to curb threats posed by fighters who could return home and prevent departures in the first place.
Pierini says authorities are pursuing several avenues, including tracking online movement and air travel.
But going after propaganda online is difficult.
Pierini describes it as a sort of whack-a-mole conundrum where when one Twitter feed is shut down, the Islamic State armed group will create 10 or 20 more.
Tracking transportation also poses problems.
For example, the revival of a passenger-name records (PNR) database for intra-EU flights would exclude other modes of transport and could result in people slipping through the cracks.
However, a major concern for all the tactics deployed with the aim of curbing foreign recruits is whether they would infringe on fundamental values of European countries.
“It basically goes against the spirit of two of the fundamental liberties in Europe – freedom of expression and the freedom of movement,” said Pierini. “It is an extremely difficult piece of work because the French and also Belgian, British and German governments, are trying be efficient in curtailing the number of people going while respecting liberties, which means essentially having enough proof that people are intending to commit a terrorist attack."
Nicholas Henin, the author of Jihad Academy who was held hostage by Islamic State fighters from June 2013 until his release in April 2014, says a variety of factors can motivate someone to join a battleground abroad.
"Many of them are extremely disappointed with their lives in the West and they are not necessarily Muslim," Henin says, adding that in the case of France 25 to 40 per cent of recruits had not come from a Muslim family.
Henin says a second group of recruits have a genuine desire to provide humanitarian aid, yet are quickly disappointed on the ground.
He adds that the US-led airstrikes has also served to the benefit of the aggressive recruitment process of the Islamic State.
"Right after the beginning of the air campaign in summer 2014 you have again a sharp increase in the departure of foreign fighters and this Western intervention against the Islamic State was also to some extent a recruitment campaign for the Islamic State," he points out.