The number of Westerners who have traveled to Syria and Iraq to fight for ISIS continues to grow, according to U.S. officials has now reached 3,400.

Many fighters heading to the ISIS caliphate travel via Turkey, and some observers have questioned the willingness or the ability of the Turkish government to intervene, said one U.S. official.

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James Clapper

In testimony Thursday, Gen. James Clapper, the director of national intelligence, told Congress the intelligence community now estimates that 3,400 citizens from Western nations have traveled to Syria and Iraq.

That's 700 more than November's estimate of 2,700 -- though officials say some portion of the higher headcount represents improved intelligence, not new recruits.

The numbers, Clapper said, are critical to understanding the potential for attacks in the West.

The U.S. intelligence community has also upped its estimate of the number of foreign fighters from all over the world, not just the West. Clapper said that ISIS now has 20,000 foreign fighters, up from 16,000 last fall, out of a total fighting force of as many as 31,000.

Clapper's also upped the number of U.S. citizens who've traveled or attempted to travel to Syria to fight for any of the groups battling the Syria government. Six months ago, the number was estimated at 100, then more recently 150. He put the number at roughly 180.

He indicated that U.S. law enforcement is monitoring those who have returned. U.S. officials have said in the past that the number of fighters currently engaged in fighting for ISIS is "around a dozen."

Officials say the ISIS appeal is broad, with at least 90 countries supplying fighters. Among them are large numbers from central Asia, including the restive Chinese province of Xinjiang, the Caucasus, and the Balkans, specifically Bosnia. A new trend, said a U.S. official, is an influx of women, particularly from the West.

"We don't break things down by gender," said the official, but the number of women is "not insignificant." ISIS has been using social media to encourage young women to join the group. In many cases, they're enticed with the promise of marriage to ISIS fighters.

U.S. agencies have differed on the overall size of the ISIS fighting force, with the intelligence community sticking with a range of 20,000 to 31,500 while the Pentagon thinks the number could be as low as 17,000.

The intelligence community's estimate is built on reports from other intelligence agencies in the Arab world and the West. Those numbers often cover different reporting periods and can be inconsistent in their specificity. France, for example, has very specific numbers. Other countries provide very general estimates.

"It's undeniable that there's been an increase in numbers," said the official, but added that as with the number of Western fighters, it's not certain how much of the rise is due to new recruits, and how much is attributable to better accounting.