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Classified data exclusively obtained by NBC News shows that the number of foreigners fighting for ISIS in Syria and Iraq has dropped by half in the past year, but those fighters who have returned to their native countries –- or have never left home -- are now launching attacks across the globe.

Earlier this month, ISIS fighters killed hundreds with attacks in Saudi Arabia, Bangladesh and Baghdad. In late June a suicide attack killed 45 people at Turkey's main international airport.

The data, which comes from a secret intelligence report recently sent to the White House, indicates that the number of foreigners fighting for the Islamic Caliphate in Syria and Iraq is now 12,000, about half what it was in early 2015.

Western countries have made it harder for their citizens to travel to the region and join the terror group. But some ISIS sympathizers are staying in their home countries and conducting operations there. And a stunning 30 percent of those who have fought in the war zone have now returned to their home countries –- at a rate of 3000 ISIS adherents per month.

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Passengers embrace outside Ataturk Airport's main entrance in Istanbul on June 28, 2016. OZAN KOSE / AFP - Getty Images

Terror expert Malcolm Nance told NBC’s Cynthia McFadden that we have entered a new phase in the war against ISIS, as the terror groups responds to its loss of territory in Syria and Iraq and depletion of troop strength by spreading the war to other countries.

“The effect that's going to happen now is like stepping on a ball of mercury,” explained Nance, head of the Terror Asymmetrics Project and a former U.S. intelligence analyst. “You step on a ball of mercury, all the pieces break up and spread around the world.”

Even so, some 500 new Western nationals travel to Iraq and Syria to fight for ISIS every month.

Based on the classified data obtained by NBC News, which does not count individuals who have died on the battlefield or returned to their home countries, 145 people from the U.S. are currently estimated to be fighting in Iraq and Syria. Despite crackdowns by European governments, 1200 French nationals are still fighting for ISIS inside the caliphate, along with 335 people from the U.K.

NBC News

By far the largest number of Westerners, however, comes from Russia. Russia and other former Soviet states account for more than 2,400 current ISIS fighters in Iraq and Syria, according to the data –- a number that has doubled in the past year. Two times that number may have made it to the caliphate, but since many, like the Chechens, are experienced fighters, they’ve been sent straight to the front lines. Their death rate is “staggeringly high,” said one senior intelligence official.

The bombers who killed 45 people at the Istanbul airport on June 28 were ISIS fighters originally from Russia’s Caucasus region.

The jihadis who took hostages at a Dhaka, Bangladesh bakery, however, were attacking close to home. All six, according to the Bangladeshi government, were Bangladeshi citizens.

“Many, many more foreigners,” said Nance, “have the ability to go home and perhaps create the circumstances for a new 9/11.”

The military pressure on the physical caliphate in Iraq and Syria, said Nance, means ISIS has to take its battle elsewhere. Said Nance, “We are creating, essentially, a ghost caliphate.”