“The people who collect tactical intelligence on the ground, day-to-day – and this isn’t Washington – but people collecting this stuff say they’re here, ISIL is here, they’re capable of striking,” Bob Baer told CNN.

Baer issued the warning after ISIL militants released a video Tuesday showing the beheading of American journalist Steven Sotloff.

US journalist James Foley, who had been missing in Syria for two years, was also executed in the same gruesome manner on camera two weeks ago.

Baer said that some American citizens who have been to Syria to fight alongside ISIL were now back in the United States by crossing the Mexican border.

US intelligence agents are aware of some people they suspect of being ISIL members and are working to gather evidence to arrest them, Baer said, adding that there are concerns about “the unknown.”

“They don’t know what their plans and intentions are. But it’s a definite concern,” the former CIA operative said.

“They’re waiting to get enough intelligence to actually run them in,” he continued. “And then there’s the unknown, of how many people have come back they’re not even aware of.”

US airstrikes against ISIL targets in Iraq have aggravated the terror group, Baer cautioned. "They have no choice in their minds but to strike back."

An estimated 12,000 foreign militants, including Americans, have traveled to Syria to fight the government there, according to the State Department.

Mike Rogers, chairman of the House Intelligence Committee, has warned that hundreds of Americans had joined ISIL and could return to the US using their American passports.

“I'm very concerned because we don't know every single person that has an American passport that has gone and trained and learned how to fight,” the Michigan Republican said on Sunday.

SHI/SHI