ISIS:Number Of People Fighting For ISIS Is More Than Three Times The Previous Estimates.

U.S. surveillance flights begin in Syria.

STORY HIGHLIGHTS

• Iraqi video apparently shows captured Saudi teen fighter
• More than 15,000 foreign fighters, including 2,000 Westerners, have gone to Syria
• The CIA says ISIS "can muster between 20,000 and 31,500 fighters across Iraq and Syria"

Baghdad (CNN) -- Just 18 years old, teenager Hamad al-Tamimi says he was a religious studies student in Riyadh before jihadists influenced and recruited him online.

Now he's in the hands of Iraqi authorities, accused of being an ISIS fighter captured in Iraq's west.

Al-Tamimi is one of numerous foreign fighters believed to have swelled the Sunni extremist group's ranks in recent months.

On Thursday, the CIA made a startling announcement: The number of people fighting for ISIS is more than three times the previous estimates.

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And among their ranks are many foreign fighters -- fighters such as al-Tamimi.

A dramatic rise

Analysts and U.S. officials initially estimated there were as many as 10,000 fighters, including those who were freed from prisons by ISIS, and Sunni loyalists who have joined the fight as the group advanced across Iraq.

But now ISIS, which calls itself the Islamic State, "can muster between 20,000 and 31,500 fighters across Iraq and Syria," a CIA spokesman told CNN on Thursday.

"This new total reflects an increase in members because of stronger recruitment since June following battlefield successes and the declaration of a caliphate, greater battlefield activity and additional intelligence," the spokesman said.

'There are many nationalities'

Al-Tamimi -- known by his nom de guerre, Abu Walid -- says he traveled from Saudi Arabia to Kuwait in July, and from there on to Turkey, a main route for jihadists into Syria.

Handlers who met him in Syria took away his passport and phone, he says. He then spent a week with, according to his estimate, some 270 fresh fighters.

"There are many nationalities," he says. "From Norway, from America, Canada, Somalia, Korea, China, Turkmenistan, Tajikistan, Uzbekistan, Egypt, Libya, Tunisia, Lebanon, and other European countries such as Germany and France."

A CIA source told CNN on Thursday that more than 15,000 foreign fighters, including 2,000 Westerners, have gone to Syria in total.

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It was not immediately clear how many have joined ISIS and how many are with other groups opposed to the Syrian government.

The foreign fighters come from more than 80 countries, the CIA source said.

Flow of fighters

Dr Mowaffak al Rubaie, a former Iraqi National Security Adviser and now a member of parliament, told CNN that thousands of Iraqis were recruited after ISIS militants seized control of the northern city of Mosul in June.

"Young Iraqis as young as eight and nine years old with AK47s, they are recruiting and brainwashing with this evil ideology," he said -- in the same way as al Qaeda in Iraq sought to recruit them in the past but on a far bigger scale.

Al Rubaie believes that Iraqis and Syrians "probably" constitute more than 70% of ISIS fighters.

They add sizable bulk to the number of foreign fighters.

"These foreign fighters when we squeeze them in Iraq and in Syria, they will permeate, they will infiltrate, they will cross the borders to Turkey, to Lebanon -- they have already started doing that and into Jordan, and Saudi Arabia is not very far," he said.

He warned that Gulf nations are "not immune" from the threat they pose. "I believe they should work very hard with us -- intelligence and cutting the oxygen from the terrorists, stopping the money, stopping the traveling of these terrorists, the young men, coming from Saudi and (Gulf nations) and North Africa."

In the cross-hairs

Recognizing the threat that ISIS poses, U.S. President Barack Obama laid out a plan earlier this week to "dismantle and ultimately destroy" ISIS, including authorizing airstrikes.

Would-be jihadists like al-Tamimi will now be firmly in the sights of the United States.

There's also been a strong diplomatic push in recent days. U.S. Secretary of State John Kerry on Thursday met with Gulf leaders in Saudi Arabia as he seeks to enlist regional support for a coalition. On Friday, he travels to Turkey.

The jihadist training

None of those he met after heading for Syria used their real names, al-Tamimi says in a Iraqi Defense Ministry video. "From Germany, I knew Abu Hamza, and from Britain one named Abu Dawoud, and from America one named Abu Ibrahim," he says. All, like him, were young.

After 22 days at a religious indoctrination camp, the Saudi says, he had to swear allegiance to ISIS leader Abu Bakr al-Baghdadi. Next followed military training at an airbase in Raqqa, an ISIS stronghold in Syria.

After a short time fighting in Aleppo, the order came to move across the essentially non-existent border into Iraq, where ISIS fighters battling Iraqi forces near the Haditha Dam -- scene of recent U.S. airstrikes and Iraqi ground operations -- needed reinforcement.

And here, less than two months after his journey began, his short-lived jihad came to an end.

A-Tamimi now says he just wants to go home. But fighters like him returning -- bringing their extremist ideology with them -- is what many nations now fear the most.

CNN.