The US Department of Homeland Security issued an intelligence assessment to law enforcement agencies Friday warning that recreational drones "could be used by adversaries" to attack the United States.

The intelligence assessment, obtained by CBS News, does not mention any known threats, but it comes amid regular reports of the small, remote-controlled drones having close calls with commercial flights and firefighting air tankers.

"The rising trend in UAS (Unmanned Aircraft Systems) incidents within the National Airspace System will continue, as UAS gain wider appeal with recreational users and commercial applications," the report said.

The report added that drones "could be used by adversaries to leverage UAS as part of an attack" and that an "emerging adversary use of Unmanned Aircraft Systems present detection and disruption challenges."