As President Obama nominates a new FBI director, the bureau is coming under rising pressure from lawmakers to explain the limits of its recently disclosed drone fleet.

Civil liberties-minded senators on both sides of the aisle have fired off sharply worded letters and statements in recent days criticizing the FBI for deploying surveillance drones without clear guidance on how to protect privacy rights.

Sen. Rand Paul, R-Ky., was the latest to scrutinize the bureau, sending a letter on Thursday to outgoing Director Robert Mueller asking a string of questions about his agency's drone use.

"I am disturbed by the revelation that the FBI has unilaterally decided to begin using drone surveillance technology without a governance policy, and thus without the requisite assurances that the constitutional rights of Americans are being protected," Paul wrote.

Mueller acknowledged Wednesday, during a Senate hearing, that the bureau has a limited number of drones that it uses for surveillance on U.S. soil. He stressed they are used in a "very, very minimal way and very seldom." Newly released documents show that the FBI has sought and received permission from the FAA at least four times to fly surveillance drones inside the U.S. since 2010.

The use of non-lethal drones for surveillance purposes is rapidly getting off the ground among local law enforcement agencies and other groups. Mueller, in acknowledging that the FBI, too, has obtained surveillance drones, said the bureau is in the "initial stages" of drafting rules and regulations for their use.

Sen. Mark Udall, D-Colo., took umbrage at that fact, saying that while drones have the potential to help law enforcement agencies, constitutional rights must come first.

"I am concerned the FBI is deploying drone technology while only being in the 'initial stages' of developing guidelines to protect Americans' privacy rights. I look forward to learning more about this program and will do everything in my power to hold the FBI accountable and ensure its actions respect the U.S. Constitution," he said in a statement.

The FBI clarified on Friday that its drone use is governed by existing bureau regulations and FAA rules. It confirmed that the specific drone model is the PUMA AE, manufactured by California firm AeroVironment. The model is hand-launched, battery-powered and can send live color and thermal video to its operator, according to the company. It is designed to respond to disasters and other incidents.

The issue is likely to follow Obama's new nominee to lead the FBI, Jim Comey. Obama nominated the former federal prosecutor on Friday.

Sen. Charles Grassley, R-Iowa, who originally asked Mueller about the drone program, also wrote a letter to Attorney General Eric Holder on Wednesday seeking "clarification" regarding a prior response from the department that did not disclose the FBI's drones. He, too, asked Holder to explain when the FBI began using them, what checks are in place for the program and whether any are capable of being armed.

FBI spokesman Paul Bresson clarified earlier this week that the aircraft are only used in "very limited circumstances to support operations where there was a specific operational need."

He cited an example of a hostage situation in Alabama earlier this year where a drone helped law enforcement. He said they are only used to conduct surveillance "on stationary subjects." And the bureau must obtain FAA approval first.