Western efforts to stem the flow of foreign fighters to the conflict in Syria and Iraq are starting to have an impact, but would-be militants are still finding their way to the battlefront.
According to the most recent estimate, more than 34,000 foreign fighters from at least 120 countries have now gone to fight in Iraq and Syria, a U.S. intelligence official told VOA on condition of anonymity. Of those, at least 6,000 are Westerners.
Those numbers are up from previous estimates issued in October 2015, when officials said there were more than 30,000 foreign fighters from at least 115 countries, including more than 4,500 from the West.
Number of US recruits unchanged
The number of Americans who have traveled or attempted to travel to Syria or Iraq "to fight or otherwise support the conflict" has remained unchanged, intelligence officials said, at approximately 250.
Officials familiar with the estimates caution that the increase in the number is likely due to several factors, including improved reporting as well as ongoing intelligence efforts to identify individuals.
Former intelligence officials say the majority of those fighters likely traveled to the battlefront following the fall of Mosul to Islamic State militants in June 2014.
"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.
He warns it has not been enough to deter would-be foreign fighters.
"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."
And while defense officials say there is growing evidence 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 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."
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 Islamic State.