“There is, of course, no such thing as a perfect weapon, and remotely piloted aircraft are no exception,” Brennan said, acknowledging that innocent civilians have been killed but describing such cases as “exceedingly rare.”

The New America Foundation, which monitors the drone campaign in Pakistan, has estimated that civilians account for between 11 percent and 17 percent of those killed. Overall, U.S. officials have said that more than 2,000 militants and civilians have been killed in Pakistan, Yemen and elsewhere since Obama took office in 2009.

Brennan cited respect for the “sovereignty” of other countries, even though a CIA drone strike in Pakistan on Sunday came just weeks after that country’s Parliament voted unanimously to demand that such operations end.

In a question-and-answer session, Brennan declined to discuss the use of signature strikes, which are based on intelligence showing suspicious behavior rather than confirmation of the location of someone on the CIA or military target list.