John Brennan said on CBS' "Face the Nation" administration officials had met at the White House Sunday to review intelligence on possible threats timed to the 10th anniversary of the Sept. 11, 2001, terrorist attacks.
He described the information -- on a possible attack in Washington or New York -- as "specific and credible" but unconfirmed.
"The president wants to make sure that we leave no stone unturned," Brennan said. "And that's what the intelligence, law enforcement and homeland security communities are doing."
He said officials have information about the "nature of the attack that is being reported" and "some information on the possible individuals" but added he didn't want to get into any "operational details."
Doing so, he said, "could compromise our ability to find out whether or not there are individuals here in the United States that are trying to carry out an attack."
Brennan said officials were pursuing a "number of leads" in "a 24/7, around-the-clock effort by all elements of the U.S. counter-terrorism community, both here in the United States as well as abroad."
New York City and Washington have been under heightened security after overseas intelligence indicated possible car bombs as well as threats to bridges and tunnels, WNBC-TV, New York, reported Sunday.
The station reported the intelligence indicated three men would travel from Pakistan to the United States to carry out an attack.
But principal White House deputy press secretary Josh Earnest told reporters no new information was reported to the president by Brennan and the gathering of senior counter-terror advisers Sunday morning.
Speaking aboard Air Force One en route to the memorial service in Pennsylvania, Earnest said, "At 8:00 a.m. this morning the president's senior counter-terrorism adviser John Brennan convened a meeting of senior counterterrorism officials in the Situation Room ... They reviewed the security preparations that have been in place this weekend, and they discussed the nature of the threat that's still out there.
"I don't have anything to report out of that meeting ... there was no specific piece of new information that was reported at that meeting that was reported to the president."
Speaking on "Fox News Sunday," Brennan said al-Qaida's ability to execute attacks has been diminished in the past decade.
"Al-Qaida ranks in terms of leadership, operatives, as well as their ability to train and deploy individuals to carry out attacks, has been seriously degraded by the constant efforts that we have been undertaking over the past decade," Brennan said. "We're doing a lot of this with our partners overseas. So their ability to carry out an attack, launch it from the areas of Pakistan or in that area has been seriously undermined."
He said officials know the terrorist group has been trying to carry out "major attacks similar to 9/11 type of attack that took place 10 years ago, but we also know that they are now trying other types of attacks, maybe smaller attacks using car bombs or other types of things."