"I wouldn't want to go into any assessments of my colleagues," Putin first told ABC's "20/20" host Barbara Walters in an exclusive interview.

But then the former KGB agent revealed his dismay over Clinton's attitude toward the terrorist threat.

"The former president of the United States ... was in a very difficult situation. But even at that time, we certainly were counting on a more active cooperation in combating international terrorism."

Putin lamented the missed opportunity, suggesting the possibility that the events of Sept. 11 might have been avoided had circumstances been different.

"I don't know whether it would have been possible to prevent these strikes on the United States by the terrorists, but it was a pity that our special services didn't get the information on time and warn the American people and the American political leadership about the tragedy that came to pass."

Since Osama bin Laden's attacks on the U.S. two months ago, Clinton has repeatedly insisted he did everything he could in the war on terrorism, though former Clinton aides admit he was frequently preoccupied with the Monica Lewinsky sex scandal in the last two years of his term.