AFP PHOTO / Saul Loeb

Committee Chairman Dianne Feinstein (D-Calif.) said in her opening statement that she planned to ask John Brennan, President Obama's nominee to lead the Central Intelligence Agency, about civilian casualties from drone strikes — one of strongest criticisms of the program by opponents. While Brennan once said he had no "confirmed" reports of civilian casualties, he later revised those remarks in the face of widespread reports on the ground, especially in Pakistan.

Feinstein asserted that civilian casualties from U.S. drone strikes now number in the single digits annually. Those numbers are difficult to know with any certainty and official U.S. estimates are secret. But some organizations do follow open-source reports on the strikes and attempt to track individual civilian casualties. According to the New America Foundation, which tracks drone strikes, the number of civilian casualties has dropped fairly dramatically in Pakistan, with no civilian deaths reported this year and only five last year in 48 strikes, compared to between 56 and 64 in 73 strikes during 2011.

And new criticism about civilian deaths has risen in Yemen, where the number of drone strikes is fast increasing. According to data from the web site Long War Journal, U.S. drone strikes in Pakistan and Yemen killed a combined 31 civilian in 2008, 84 in 2009, 20 in 2010, 30 in 2011, and 39 civilians in 2012.

Max Fisher contributed to this report.