With more than 20,000 foreigners known to have joined ISIS, the U.S. intelligence community is increasingly concerned about American citizens who decide to fight alongside the terrorist group, CNN reported.

Charges against two men accused of aiding the organization, which has imposed its brutal rule on the large swaths of territory it controls in Iraq and Syria, were released on Wednesday, the news channel detailed.

Authorities say a 21-year-old California man, Adam Dandach, tried to travel to Syria to join the self-styled "Islamic State," while a 17-year-old Virginia student supported the group's recruiting efforts. The incidents are the latest in what CNN called "a growing number of similar cases (in) the United States."

The FBI said Dandach attempted to offer his services to ISIS, hoping to work under the direction and control of the terrorist organization; his arraignment is scheduled for later this month.

The Virginia resident, whom authorities did not name because of his age, is accused of recruiting for ISIS, distributing the group's messages and helping a slightly older individual to travel to Syria, where the adult is believed to have joined its fighters.

Some 180 Americans have tried to join the fight in Syria, where ISIS is part of a bloody civil war against more moderate rebels and forces loyal to dictator Bashar al-Assad, according to National Intelligence Director James Clapper.

The FBI and the U.S. Department of Homeland Security have sent a joint warning to local law-enforcement agencies, advising them that a growing number of young Americans are offering themselves up to fight with ISIS, CNN affiliates said in another report.

The warning detailed some of the reasons that incentivize citizens to join the Islamist organization: boys are usually motivated by anti-U.S. sentiments, while girls pursue their goal of becoming Islamic brides in what seems to be an idealized notion of what life is like in Syria, the document noted.

Americans across the board, meanwhile, seem to warm to the idea of challenging ISIS on the ground -- and not just through the airstrikes currently being carried out by the United States and its allies, MSNBC revealed. Sixty-two percent of voters "support ... sending U.S. ground troops to fight ISIS in Iraq and Syria, with strong support across all party, gender and age groups," Quinnipiac University national poll noted.