Civilian drones pose a "steadily escalating threat," as the devices are likely to be used by terrorists, criminal groups or drug cartels to carry out attacks in the U.S., FBI Director Chris Wray told a Senate committee.

"Terrorist groups could easily export their battlefield experiences to use weaponized" drones, Wray said in written testimony for a hearing Wednesday of the Senate Homeland Security and Governmental Affairs Committee.

Wray's comments, his most forceful to date on the threat from drones, come as U.S. law enforcement and homeland security agencies have just obtained legal authority to monitor drone communications and to disable them in extreme cases as a result of such concerns. The anti-drone measures were contained in a bill setting aviation policy that President Donald Trump signed on Friday.

"The FBI assesses that, given their retail availability, lack of verified identification requirement to procure, general ease of use, and prior use overseas," drones will be used in an attack in the U.S. including "a mass gathering," Wray said.

The rapidly expanding civilian drone market has prompted optimism it will be an economic boon for farmers, utilities and delivery companies, and the Trump administration in May unveiled 10 test beds for the new technology. The government predicts the total U.S. fleet of small drones will reach 1.6 million this year.

At the same time, however, growing concerns about criminal and terrorist use of the devices have slowed the government's plans for broader flight rules.

The Federal Aviation Administration had planned by the end of 2016 to propose a framework for allowing routine flights over people for the first time. That action was blocked after the FBI and the Department of Homeland Security raised concerns. As a result, the government is drawing up regulations that will require most of the devices to broadcast their identity and location so that they can be tracked by authorities.

The FBI has observed "repeated and dedicated" attempts by Islamic State, also known as ISIS, and al-Qaida to use drones in attacks, Wray said. Criminal gangs such as MS-13 and Mexican drug cartels have also used the devices, he said. Drones apparently were used in a recent unsuccessful attempt to assassinate Venezuela President Nicolas Maduro.