New figures on the flow of foreign fighters to Syria and Iraq show the scope of the problem is much bigger than first thought, with would-be militants still finding their way to the battlefront.
For the second time this week, intelligence officials have released updated estimates on the number of foreign fighters, now saying more than 36,500 from at least 120 countries have gone to take part in the fight since the start of the conflict.
That figure includes at least 6,600 Westerners.
The earlier estimates had put the total number of foreign fighters at more than 34,000 from 120 countries, including at least 6,000 Westerners.
Back in October, intelligence officials had estimated there were more than 30,000 foreign fighters from at least 115 countries, including more than 4,500 from the West.
FILE - Two men accused of fighting with the co-called Islamic State militant group hide their faces before their trial in Germany November 30, 2015.
Number of US recruits unchanged
Despite the increase in the total, the number of American fighters has remained unchanged since the October estimates, intelligence officials told VOA on condition of anonymity.
They said approximately 250 Americans have traveled or attempted to travel to Syria or Iraq "to fight or otherwise support the conflict." Of those, about 24 have been killed in Iraq or Syria.
Officials familiar with the estimates caution that the increase does not indicate a current surge in foreign fighters, but is likely due to improved reporting as well as ongoing intelligence efforts to identify individuals.
"It's tapered off because it's physically gotten harder there and nations are cracking down more, not just the U.S.," said Patrick Skinner, a former intelligence officer now with The Soufan Group, a strategic security intelligence consultancy.
Skinner said a majority of foreign fighters likely traveled to the battlefront following the fall of Mosul to Islamic State militants in June 2014; but, he also warned it has proven difficult to shut down the flow.
"As long as people are willing to go there, they can get there," Skinner said. "We've overestimated our ability to detect and disrupt extremist travel."
The foreign fighter issue has alarmed some U.S. lawmakers.
"The threat is increasing, not decreasing," House Homeland Security Committee Chairman Michael McCaul told reporters Friday. "That number was 25,000 last year."
"I don't think the strategy is working," he added.
While defense officials say there is growing evidence the Islamic State is being forced to turn to conscription, the group continues to be buoyed by a steady influx of recruits — enough to maintain a constant force of 20,000 to 30,000 fighters.
Through the end of 2015, slightly more than 1,000 foreign fighters a month were joining the Islamic State, a U.S. official familiar with the data told VOA. The official also said the most popular route continued to be crossing into Syria from Turkey, despite Turkish efforts to crack down along the border.
“All of it is done through smuggling networks in Turkey, operating though Turkey, in Iraq and certainly in Syria,” Mubin Shaikh, an ex-security and Canadian counterintelligence operative, told VOA via Skype.
“There’s some attention on the networks,” he said. “But there really isn’t a focus on attacking the networks and making that the framework in which the defeat of ISIS is supposed to be applied.”
Spanish police arrest a man, left, accused of collaborating with the Islamic State, after searching a cybercafe in Mataro near to Barcelona, Dec. 8, 2015.
Young people, women drawn to jihad
There is also evidence that the jihadist-foreign fighter message continues to resonate with European youth and with women despite counter-messaging efforts.
“It’s still a profile that tends to be quite young,” said David Sterman with the New America International Security program.
“The average age is 24 — many teenagers within the sample,” he said. “Women continue to be quite well represented.”
U.S. counterterrorism officials have also worried that Russia’s entry into the Syrian conflict on behalf of Syrian President Bashar al-Assad would spur more would-be militants to join the fight.
“It would not be surprising if ISIL features the Russian build-up as a tie into their apocalyptic narrative, and to help bridge the generational divide among jihadists with Moscow’s actions in Afghanistan and Syria as bookends,” a U.S. counterterrorism official said at the time, using an acronym for the Islamic State.