WASHINGTON - Former Defense Secretary Robert Gates said drone strikes against U.S. citizens should be subject to independent review, throwing his support behind an effort to restrict presidential action when it comes to killing American citizens.

Gates said on CNN's "State of the Union" program on Sunday that he believes the Obama administration has applied stringent safeguards in the targeted killing of U.S. citizens suspected of terrorism "but who is to say about a future president?"

The issue, which received little attention during President Obama's first term, became a central focus during the Senate confirmation hearing of John O. Brennan, nominated to be the new chief of the Central Intelligence Agency. Both Republicans and Democrats continued to press the issue on the Sunday morning talk shows.

"The president, a politician, Republican or Democrat, should never get to decide someone's death by flipping through some flashcards," Sen. Rand Paul, R-Ky., said on the CNN program.

Senators last week challenged Brennan about the legality of targeted drone strikes against U.S. citizens linked to Al-Qaeda, including questioning the legal foundation the administration applied. That basis became public in a set of Justice Department memos to the committee handed over just hours before the hearings began.

Democrats weighed in as well, insisting that oversight of the drone strikes, which have proven one of the most successful weapons against the leadership of various terrorist organizations, isn't a partisan issue.

"I've looked into this and haven't found one public hearing on drones," Rep. Keith Ellison, D-Minn., said on ABC's "This Week" program.

"I'm glad the president invited the conversation," Ellison said. "I think we ought to take him up with it." He was referring to the administration's vow to discuss the legal basis for strikes, including the release of documents.