Senator Rand Paul, a Kentucky Republican, delayed a vote on Brennan with a filibuster over the possible use of drones against American citizens on U.S. soil.
Attorney General Eric Holder told Paul in a March 4 letter that "the U.S. government has not carried out drone strikes in the United States and has no intention of doing so."
Last month, Paul introduced legislation to prevent "unwarranted government intrusion" by drones.
The bill proposes that law enforcement officers be prohibited from using drones to gather surveillance or collect evidence without a warrant, unless there is an imminent danger to life or a high risk of a terrorist attack.
The measure also makes an exception so that drones can be used to patrol the nation's borders. The Senate has not taken up Paul's proposal. A similar one was previously introduced in the House.
Drone use more common
Unmanned drone use is becoming more common in the United States although it is not lawful in many cases.
The FAA forecasts some 10,000 civilian drones will be in use in the United States within five years, including those for law enforcement and commercial purposes.
Amie Stepanovich, of the Electronic Privacy Information Center, has previously said law enforcement should not use drones as an alternative to police patrols.
She said that they should be used for specific operations and that Congress should pass a law requiring legal permission.
Because they are cheaper to use than helicopters, unmanned aircraft can be used to monitor crops and livestock, look at damage to buildings and for other uses.
The FAA recently announced plans to create six drone test sites around the country.