A new book on the 2012 presidential campaign claims that President Barack Obama told aides that he is "really good at killing people."

According to Mark Halperin and John Heilemann, the authors of Double Down: Game Change 2012, Obama made the comment while discussing drone strikes last year. CNN's Peter Hamby noted the anecdote in his review of the book for the Washington Post.

While the White House has not commented on the president's alleged remarks, senior Obama adviser Dan Pfeiffer on Sunday brushed off, but did not dispute, other reports from the book, including that campaign officials weighed replacing Vice President Joe Biden with former Secretary of State Hillary Clinton on the Democratic ticket.

"The president is always frustrated about leaks," Pfeiffer said on ABC's "This Week." "I haven't talked to him about this book. I haven't read it. He hasn't read it. But he hates leaks."

The quote comes in the context of both the drone program and the killing of Osama bin Laden by a special forces strike force. The passage also specifically references the death of another al Qaeda leader, Anwar al-Awlaki, who was killed by a CIA drone strike in Yemen on Sept. 30, 2011.

Obama didn't need to run through this preamble. Everyone knew the litany of his achievements. Foremost on that day, with the fresh news about al-Awlaki, it seemed the president was pondering the drone program that he had expanded so dramatically and with such lethal results, as well as the death of Bin Laden, which was still resonating worldwide months later. "Turns out I'm really good at killing people," Obama said quietly, "Didn't know that was gonna be a strong suit of mine."

Al-Awlaki's 16-year-old son, Abdulrahman, who was an American citizen, was killed in a separate drone strike two weeks after his father.

"My grandson was killed by his own government," the teenager's grandfather Nasser al-Awlaki wrote in a New York Times op-ed in July. "The Obama administration must answer for its actions and be held accountable."

Obama, who was awarded the Nobel Peace Prize in 2009, has overseen the expansion of the CIA's targeted killing program, which the Bureau of Investigative Journalism estimates has killed between 2,528 and 3,648 individuals in Pakistan since 2004. That organization also estimates that between 416 and 948 of those killed in drone strikes were civilians -- an estimate disputed by the Obama administration.

Among those civilians, according to Amnesty International, was a Pakistani grandmother killed alongside 18 civilian laborers in a 2012 strike. The grandmother's family came to Washington, D.C., last month to testify before Congress and urge an end to drone warfare.

Despite the president's pledge to be more transparent about the drone program, the administration has continued to face criticism for its secrecy on the legal case for the strikes.

The Huffington Post's Matt Sledge reported last month that a coalition of human rights and journalism groups is putting pressure on the administration to release the opinions that underpin the program.

"While the government has an obligation to protect properly and appropriately classified information, democracy does not thrive when our national security programs and the intelligence community's actions are shrouded in secrecy," the groups wrote in the letter. The groups hope the Obama administration will take the concrete step of instructing the Justice Department's Office of Legal Counsel and the Foreign Intelligence Surveillance Court to release the legal opinions that provide the foundation for the U.S.' drone war and the NSA's surveillance operations.

In a May speech at the National Defense University, Obama defended the use of drones.

"Let us remember that the terrorists we are after target civilians, and the death toll from their acts of terrorism against Muslims dwarfs any estimate of civilian casualties from drone strikes," Obama said.