Washington: Locked down in his Abbottabad hideout which he never left in six years, Osama bin Laden followed "absolutely phenomenal operations security", and was taken by surprise when attacked by the US forces, a top American counter-terrorism official has said.

Virtually giving a clean chit to Pakistan's authorities, John Brennan, Assistant to the US President for Homeland Security and Counterterrorism, said in his speech at the Paul Nitze School of Advanced International Studies that the al Qaeda chief maintained high level security and secrecy at his hideout.

"He (bin Laden) was there for six years. To our knowledge, he never left that compound once he got there. The people that were out of the compounds were, most of them never left either. You have a couple of folks who basically served as his interlocutors with the outside world and provided the type of support that he needed," he said.

Brennan said Osama was relaxed in his Abbottabad hideout and confident about his security there.

However, he added that this does not mean there weren't some elements in Pakistan which knew about bin Laden's presence.

"So this is something that I don't know how many people in al-Qaeda, in the senior leadership ranks, knew exactly where bin Laden was. But he is someone who knew that if he... let it be known of his whereabouts... his days were going to be numbered. I think he was -- he became, then, confident and relaxed in Abbottabad. I think he was totally taken by surprise when our brave SEALs confronted him that night," Brennan said. Brennan said the Pakistani leadership too was taken "totally unawares" by the fact that bin Laden was found in a compound in Abbottabad, close to a military academy and not too far from the nation's capital.

"They are trying to figure out why, in fact, he was able to be there for six years. That's not to say that there aren't elements somewhere within the Pakistani broad establishment that were knowledgeable, provided assistance. But you know, looking at that situation, bin Laden and the people at that compound practiced absolutely phenomenal OPSEC (operations security)," Brennan responded to a question.

The al Qaeda chief was killed in a secret US raid on his compound in Abbottabad on May 2.