FBI Director Christopher Wray warned Congress on Wednesday that terrorist groups may soon have the capacity to carry out drone attacks against the United States and described the threat as imminent.

"We do know that terrorist organizations have an interest in using drones; we've seen that overseas already with some growing frequency and I think the expectation is it's coming here imminently," Wray testified before the Senate Homeland Security and Government Affairs Committee. "I think they are relatively easy to acquire, relatively easy to operate, and I think quite difficult to disrupt and monitor."

Nicholas Rasmussen, director of the National Counterterrorism Center, said terrorists could target American citizens using drones equipped with chemical weapons or small explosives the size of a grenade.

To prepare for the threat, he said, counterterrorism agencies have established teams of intelligence experts who are dedicated full-time to examining the possible tactics and techniques of terrorists in possession of drones.

"Two years ago this was not a problem. One year ago this was an emerging problem. Now it's a real problem, and so we are quickly trying to up our game," Rasmussen testified Wednesday.

U.S. Central Command told Fox News in April it had seen an uptick in the Islamic State's use of weaponized and surveillance drones against Iraqi and American forces in Iraq and Syria over a six-month span beginning in October. At the time, CENTCOM recorded as many as 30 encounters per week between coalition troops and unmanned aerial vehicles.

ISIS first acquired commercial drones two years ago for surveillance purposes, but has since demonstrated a mounting desire to use the technology in attacks against U.S.-led coalition forces.

The militant group earlier this year announced the creation of an "Unmanned Aircraft of the Mujahideen" unit made up of bomb-equipped drones intended to present "a new source of horror for the apostates!" In its official al-Naba newsletter, ISIS said its drones had killed nearly 40 Iraqi troops in one week, though the Washington Post dismissed the claim as "almost certainly exaggerated."

ISIS released a propaganda video in February showing jihadists using a drone to drop a bomb on an Iraqi army outpost. The blast scattered Iraqi troops and appeared to leave one person either dead or wounded.

While the effect of ISIS drones on U.S. and Iraqi troops has so far been minimal, American officials have said the terrorist group seems to have crossed a threshold in its ability to maneuver the unmanned aircraft.