"This administration has decided that they don't want to do detention any more -- the [George W.] Bush administration got in trouble with detention -- so they're now going to just kill people," said John Bellinger, who drafted the Bush administration's legal framework for targeted drone killings.
"So instead of detaining members of al-Qaida, they're killing members of al-Qaida," Bellinger told a conference at Washington's non-profit Bipartisan Policy Center.
The White House had no immediate comment.
Obama said Tuesday he would recommit himself to closing the Guantanamo Bay detainment and interrogation facility at the U.S. Naval Base in Cuba.
"It's not sustainable," Obama said at a White House news conference. "The notion that we're going to continue to keep over a hundred individuals in a no man's land in perpetuity ... the idea that we would still maintain forever a group of individuals who have not been tried -- that is contrary to who we are, it is contrary to our interests, and it needs to stop."
Guantanamo holds 166 detainees, despite 86 of them having been cleared for release. About 100 prisoners are on hunger strike, with about 21 fed through nasal tubes.
The inmates are protesting their indefinite detention without charges or trials.
Bellinger, a lawyer focusing on national security and public international law, accused the Obama administration of over-reliance on drone attacks because imprisoning suspected terrorists in Guantanamo is too problematic.
At the same time, Bellinger told the conference he believed the administration was justified under international law in pursuing targeted killings.
Under Obama, the use of targeted killings has expanded, largely through combat drones operating in Afghanistan, Pakistan and Yemen.
The killings began after the Sept. 11, 2001, terrorist attacks under a U.S.-led military campaign to eliminate al-Qaida and other militant organizations.
Bellinger said the big issue was not the legality of targeted killings but rather acceptance by the international of Washington's so-called global war on terrorism.
"If we are in a real war -- if we're at war with Germany or Japan in World War II -- and we have developed a weapon in which one could only kill a single person rather than engage in mass bombings in Dresden [Germany], everybody would say: 'That's wonderful! That's legal. That's good.'
"So it's not actually that targeted killings are bad. Targeted killings, when they are lawful and legitimate, can be good," said Bellinger, who was a State Department legal adviser from 2005 to 2009 and a National Security Council legal adviser from 2001 to 2005.
"The issue really here ... is that there is a fundamental disagreement around the world, which I experienced when I was the legal adviser, as to whether the United States really is in a war at all. And we are about the only country in the world that really thinks that we are in an armed conflict with al-Qaida."
The State and Defense departments had no immediate comment.
An estimated 4,700 people have been killed by some 300 U.S. drone strikes in four countries, British newspaper The Guardian said.
"These drone strikes are causing us great damage around the world," Bellinger told the conference.
"On the other hand, if you're the president of the United States and ... your CIA director is giving you information that suggests that there are threats against you, that another 9/11 could happen, and you do nothing, then you also have a problem," Bellinger said.
He added he was sure the Obama administration "is mindful of" the 2004 report by the National Commission on Terrorist Attacks Upon the United States, also known as the 9/11 Commission, that "suggested not enough had been done [to prevent the Sept. 11, 2001, terror attacks] and is mindful of that."