WASHINGTON (CBS DC) — The FBI and the Department of Homeland Security have issued a joint warning to U.S. law enforcement agencies that a growing output of Islamic State of Iraq and Syria recruiting efforts are trending with American youth.

Sacramento FBI officials say ISIS propaganda is inspiring Western youth to travel to Syria to serve as fighters with the ISIS militant group. Recruitment efforts are increasingly issued through social media engagement – something the federal law enforcement agencies want people to report if they see “suspicious activity.”

Thousands of Westerners, including large amounts of teenagers, are believed to be fighting alongside ISIS militants, along with scores of others who have been arrested en route to the region.

Some of the more high-profile ISIS recruitment efforts include three teenage girls from Denver, sisters aged 15 and 17 and a 16-year-old friend, who were stopped in Germany last October, reportedly on their way to Syria. And Mohammed Hamzah Khan, 19, was arrested at Chicago’s O’Hare airport in October and has plead not guilty to attempting to provide material support to a foreign terrorist organization.

Nineteen-year-old Shannon Conley of suburban Denver was sentenced in January to four years in prison after she plead guilty to one count of conspiracy to provide material support to a foreign terrorist organization.

The FBI’s website illustrates the bureau’s concerns that ISIS influence is taking hold domestically.

The day after a criminal complaint was unsealed in federal court in New York charging three Brooklyn residents with attempting to provide material support to ISIS, FBI Assistant Director Michael Steinbach briefed a congressional subcommittee on the dynamic threat posed by foreign fighters traveling in support of ISIS and the continued threat to America posed by homegrown violent extremists, according to FBI.gov.

The Bureau estimates upwards of 150 Americans have traveled or attempted to travel to Syria to join extremist groups, and it also has to consider the influence of groups like ISIS on individuals located in the U.S. who could be inspired (particularly through the Internet and social media) to commit acts of violence.

“It is this blending of homegrown violent extremism with the foreign fighter ideology that is today’s latest adaptation of the threat,” said Steinbach. “Regular engagement with our domestic and foreign partners concerning foreign fighters is critical” at each of the bureau’s 56 field offices for the Joint Terrorism Task Force.

CBS News foreign correspondent Clarissa Ward spoke with two Western jihadists – an American and a Dutchman – to find out why they would join the fight with Islamic State terrorists.

“I don’t hate America,” said Ibn Zubayr, who joined the al Qaeda-linked Jabhat al Nusra rebel group. “That’s my home. That’s where I grew up. I don’t have a need to hate America itself. But the government and their policies as far as the Muslim lands, that’s another story.”

Speaking at a February global summit aimed at fighting violent extremism, President Barack Obama said he wants to shut down ISIS’ social media propaganda machine. ISIS reportedly controls 90,000 Twitter accounts in addition to thousands of other social media profiles.

“The high-quality videos, the online magazines, the use of social media, terrorist Twitter accounts — it’s all designed to target today’s young people online,” Obama said.