US President Barrack Obama has told his top aides during the 2012 presidential campaign that he is " really good at killing people ", according to a new book.The book titled “Double Down: Game Change 2012,” is written by journalists Mark Halperin and John Heilemann. According to the book, the US president made the remarks while discussing the country’s controversial assassination drone strikes in several Muslim countries. The White House has not yet commented about the alleged remarks. On Sunday, a top advisor to President Obama, however, rejected the other leaks in the book including the alleged consideration by Obama aides to replace Vice President Joe Biden with Secretary of State Hillary Clinton during the 2012 presidential campaign. "I can tell you, no one in the campaign as well as the president ever serious considered this in any way shape or form," said Dan Pfeiffer while speaking on ABC's "This Week". Pfeiffer added that he has not read the book and has not talked with Obama about it yet but he knows that “the president is always frustrated about leaks.” The US president who won the Nobel Peace Prize in 2009 has expanded Washington’s drone strikes which are carried out in several Muslim countries, including Pakistan, Afghanistan and Yemen, during his presidency. According to a 2012 New York Times report, Obama personally approves every name on a secret “kill list” which is used in the targeted killing operations. The White House says its drone war is aimed at killing suspected militants but local media reports indicate civilians are the main victims of the attacks.