WASHINGTON — Agonizing "minutes passed like days," after Barack Obama made what a top aide dubbed the "gutsiest" presidential call in years, and followed in real time as special forces swooped on Osama bin Laden.

Obama's gruff anti-terror advisor John Brennan, who hunted the Al-Qaeda mastermind for 15 years, offered tantalizing details Monday of the high-stakes operation in the Pakistani city of Abbottabad in which bin Laden died.

"It was probably one of the most anxiety-filled periods of time, I think, in the lives of the people who were assembled here yesterday," Brennan told reporters in a colorful White House briefing.

"The minutes passed like days, and the president was very concerned about the security of our personnel.

"That was what was on his mind throughout, and we wanted to make sure that we were able to get through this and accomplish the mission."

Brennan would not say exactly how Obama and his top advisors were able to follow Sunday's 40-minute Navy SEAL operation unfolding in real time -- but the suspicion was that some kind of sophisticated communications technology was available to them.

"It was clearly very tense, a lot of people holding their breath," Brennan said. "And there was a fair degree of silence as it progressed as we would get the updates.

"When we finally were informed that those individuals who were able to go in that compound and found an individual that they believed was bin Laden, there was a tremendous sigh of relief."

Brennan was also asked several times whether the national security team had been able to hear exchanges of fire in the compound where bin Laden was sheltering but demurred.

"We were able to monitor the situation in real time," was all he would say.

Had the sophisticated helicopter-borne operation gone wrong, the stakes for Obama would have been huge, and the operation could have dealt him a political blow from which he would not recover as he seeks reelection next year.

Given the downside risk, Obama faced the kind of lonely decision that only the presidents who have made such perilous calls before can understand.

"The president had to evaluate the strength of that information and then made what I believe was one of the most gutsiest calls of any president in recent memory," Brennan, a former senior CIA officer, said.

Brennan said the most anxious moment of the operation came when one of the helicopters carrying Navy SEALS special forces developed technical problems after flying into the compound.

"When that helicopter was seen to be unable to move, all of a sudden, you had to go into Plan B. And they did it flawlessly," said Brennan, describing the moment when another helicopter was called into the breach.

Brennan also revealed that the United States did not inform the Pakistanis of the operation until its helicopters had exited Pakistani air space, and of fears that Islamabad would scramble fighters to intercept the mission.

And he even raised suspicions that US forces might have been prepared to take action against any Pakistani effort to intercept their helicopters, saying the US was "clearly" concerned the Pakistanis might decide to scramble jets.

"So we were watching and making sure that our people and our aircraft were able to get out of the Pakistani airspace, and thankfully there was no engagement with Pakistani forces," Brennan said.

"Thankfully no Pakistani forces were engaged, and there were no other, you know, individuals who were killed aside from those on the compound."

Brennan also related how the realization dawned gradually that US special forces had killed bin Laden and were able to get out alive along with his remains.

"It was more of a growing sense of confidence and a growing sense of accomplishment," he said.

Asked for Obama's reaction, Brennan replied: "'We got him.'"