WASHINGTON - US Vice President Joe Biden said on Tuesday he advised President Barack Obama in May 2011 to go ahead with the raid that killed al-Qaida leader Osama bin Laden, contradicting his past comments about the discussions leading up to the operation."As we walked out of the room and walked upstairs, I said - I told him my opinion that I thought he should go, but follow his own instincts," Biden said during an event at George Washington University."I never ... say what I think finally until I go up in the Oval (Office) with him alone."Biden, whose comments came as he weighs whether to enter the 2016 presidential race, has previously said he advised Obama against approving the 2011 raid without taking further steps to verify that bin Laden was present at the Pakistan compound where he was eventually found."I said, 'Wait another seven days for information'" Biden said at a fundraising event in New Hampshire on May 26, 2011, according to reports in The New York Times and other media.Hillary Clinton, the Democratic front-runner in the 2016 presidential race, who was Obama's secretary of state at the time, has highlighted her support for the raid. Biden avoided naming her among those who spoke forcefully in favor of the operation going forward."There were only two people who were definitive and were absolutely certain," he said. Central Intelligence Agency Director Leon Panetta said "go," Biden said, and Secretary of Defense Robert Gates said "don't go."Clinton in the Democratic debate last week took a veiled shot at Biden by emphasizing her involvement in Obama's decision to authorize the raid.