Ahead of the 15th anniversary of 9/11, former secretary of state and Democratic presidential nominee Hillary Clinton said going after al Qaeda leader Osama bin Laden into Pakistan was "not an easy choice".

"I was part of the small group advising President Barack Obama whether or not the intelligence we had was good enough to take a chance, to go deep into Pakistan, to try to finally bring Osama bin Laden to justice. It was not an easy choice by any means," Clinton said at an election rally in North Carolina.

"That's why who sits at the head of that table in the situation room has to be able to sort out fact from opinion, has to be able to ask the hard questions, pursue even the most difficult leads," she said giving an insight into the May 2, 2011 killing of Osama, the 9/11 mastermind, in a US military operation at his hideout in Abbottabad.

Her comments came before the 15th anniversary of the al Qaeda attack on the twin towers in New York that killed more than 3,000 people, resulting in the US invading Afghanistan.

"We went through that hour after hour after hour. And then the president went around the table asking each of us what we advised. And we were split because it was not some kind of easy layup. I believed it was strong enough that we needed to take action and I supported taking action that would determine whether or not we were successful. That meant sending in special forces," she said.

"Now, you know what happened. I was in that Situation Room watching that day. The most stressful 30 minutes of my life probably. Because you remember one of the helicopters nicked.