A teenager from the suburbs of the US city of Chicago has been arraigned on terrorism charges in connection with an FBI sting operation that has raised new questions about whether US investigators are engaging in entrapment.

Abdella Ahmad Tounisi, 18, made a brief court appearance Tuesday in federal court in Chicago.

The American-born man from the Chicago suburb of Aurora is accused of seeking to join al-Qaeda-affiliated Jabhat al-Nusra, which is fighting Syria's government in a civil war.

He tried to join the group through a website, constructed by the FBI, which urged readers to "join your lion brothers... fighting under the true banner of Islam".

Critics have said the use of such sites raises questions about whether authorities are overreaching, wooing impressionable youth to contemplate crimes that otherwise wouldn't cross their minds.

"These sites can end up creating crimes," said Phil Turner, a former federal prosecutor turned defence attorney in Chicago.

"Real terrorists don't need to go to a website for contacts. They have real contacts."

Federal investigators, he added, sometimes favour internet stings because they are less costly and labour intensive than traditional stakeouts.

'Jihad' calls

Authorities have noted that visiting such sites or fantasising about acts of terrorism is not the crime; rather, it is acting on those fantasies.

Tounisi took steps, authorities said, which included trying to board a plane in Chicago.

He was arrested at O'Hare International Airport on Friday as he prepared to start the first leg of a trip that authorities allege he hoped would hook him up with fighters in Syria.

Tounisi is charged with one count of attempting to provide material support to foreign terrorists. If convicted, he faces a maximum 15-year prison term. His attorney, Molly Armour, declined comment on Tuesday.

In an interview with Al Jazeera, Peter Neuman, a professor at King's College in London said, FBI operations like the one that snagged Tounisi have been controversial, because "they are essentially picking people at the point where they are simply expression interest".

"They haven't done anything, and then the FBI makes them do something," Neuman said. "I would say that in many cases, that's quite questionable".

'No fighting skills'

Despite his orange jail garb and shackled ankles, Tounisi looked younger than his 18 years at his court appearance, during which a judge delayed a decision on bond.

It was only months ago that he is accused of coming across the website, which not only offered to connect would-be fighters with terrorists, but also offered advice on how users could cover their tracks from law enforcement, according to a detailed federal complaint.

At some point, Tounisi allegedly took the bait. In an April 1, 2013, email to an FBI agent posing as a recruiter, Tounisi comes across as earnest and frank.

"Concerning my fighting skills, to be honest, I do not have any," he allegedly wrote.

While he is accused of taking steps to further his desire to fight in Syria, his online searches also seem to convey some concern about legal implications.

According to the complaint, for instance, he searched the phrases "providing material support what does it mean" and "Terrorism Act 2000."

Joan Hyde, a spokesman for the FBI in Chicago, declined any comment Tuesday, saying the agency does not comment on ongoing cases.