WASHINGTON (AFP) - US authorities on Thursday issued an appeal to police and the public to remain "vigilant" against attempts by the Islamic State in Iraq and Syria (ISIS) group to recruit young Americans online.

The warning from the Federal Bureau of Investigation (FBI) and the Department of Homeland Security (DHS) came after the reported arrest last week of a 17-year-old boy who federal authorities allege helped another young man travel to Syria to join forces with the ISIS militants.

The "bulletin" from federal authorities was designed "to provide further information on the continuing trend of Western youth being inspired by ISIL to travel to Syria to participate in conflict," said a DHS official, using an alternative acronym for the extremists.

"We remain concerned about the recruitment efforts made by ISIL particularly through social media engagement and we urge the public to remain vigilant and report any suspicious activity to law enforcement," the official, who spoke on condition of anonymity, told AFP.

In the case of the teenager arrested last week, The Washington Post reported Wednesday that FBI agents raided the boy's family home in Woodbridge, a Washington suburb in northern Virginia, and led him away in handcuffs.

Authorities believe the teenager helped a man not much older than himself make his way to Syria, partly by using online contacts linked to the ISIS militants.

Western governments and Arab allies have vowed to counter the ISIS group's propaganda on social media. And Twitter has come under pressure to remove accounts used by ISIS and similar extremist groups.