“People, including Americans, can go to Syria and learn about dangerous techniques, and it's easy to get in and get out,” James Comey told reporters on Thursday in a press conference.

“My concern is that people can go to Syria, develop new relationships, learn new techniques and become far more dangerous, and then flow back” to the US, he added.

According to The New York Times, at least 70 Americans have either gone to Syria or tried to go there in order to fight alongside insurgents against the Syrian government.

Washington is now worried that al-Qaeda-linked groups might have recruited and trained Americans who have gone to Syria in order to get them to carry out attacks when they return to the US.

European officials are sharing the same worries as analysts say at least 1,200 Europeans have gone to Syria to fight against the Syrian government.

According to US officials, most of the Americans who have gone to Syria are still there while a few have been killed.

Thirty three-year-old Nicole Lynn Mansfield of Flint, Michigan, was killed last May while fighting alongside insurgents in Idlib Province.

A former US Army soldier from Phoenix, Eric G. Harroun, who was accused of fighting alongside an al-Qaeda-affiliated group in Syria, was also indicted by a federal grand jury in Virginia last year.

“We know al-Qaeda is using Syria to identify individuals they can recruit, provide them additional indoctrination so they’re further radicalized, and leverage them into future soldiers, possibly in the US,” a US intelligence official, who spoke on the condition of anonymity, told the Times.

Syria has been gripped by deadly unrest since 2011. According to reports, the Western powers and their regional allies -- especially Qatar, Saudi Arabia, and Turkey -- are supporting the militants operating inside Syria.

NTJ/BA