 -- The leader of al-Qaeda appeared in a video online today, ahead of the 15th anniversary of Sept. 11, calling the deadly attack a "slap" to the U.S. and its allies.

"We mark in these days the passage of nearly 15 years since the blessed invasions in Washington, New York and Pennsylvania," Ayman al-Zawahiri says in the video, according to a translation by the SITE Intelligence Group.

Al-Zawahiri later threatens the U.S. again, saying that "as long as your crimes continue, then the events of Sept. 11 will be repeated a thousand times, Allah permitting."

The video is part of al-Qaeda's recent push to remain relevant in a world in which a rival terrorist organization, ISIS, draws more attention and far more recruits, according to former White House counterterrorism adviser Richard Clarke.

"The fact that he is communicating on the anniversary is an attempt by him to remind everybody that he exists and is the nominal head of [al-Qaeda]," said Clarke, now an ABC News consultant.

CIA Director John Brennan said in a recent interview that while al-Qaeda has been "diminished" through aggressive counterterrorism efforts, the group is "still a very serious concern and threat, and the core of al-Qaeda -- Zawahiri and others of that ilk -- still think of the West as the major enemy."

"So I do believe that they consider the United States to be a principle target," Brennan told the Combating Terrorism Center at West Point.

Elsewhere in the video, al-Zawahiri appears to make a veiled criticism of ISIS, saying that "real soldiers ... do not impose themselves on you as rulers without your acceptance and consultation."

Nada Bakos, a former CIA officer who tracked al-Qaeda in Iraq in the mid-2000s, said al-Zawahiri's message appeared to be in part reflective of the "competition" between al-Qaeda and ISIS.

"All of this [9/11 talk] is very much 'Look what we were able to pull off. No one else can do anything like this,'" Bakos told ABC News. "He's really trying to present his case [for al-Qaeda]."

Clarke said the video also indicates that al-Zawahiri has learned from the security mistakes of Osama bin Laden. Despite being wanted by the U.S. since the late 1990s and a $25 million reward for information leading to al-Zawahiri's capture, he has managed to elude authorities.

Clarke said he suspects al-Zawahiri is hiding somewhere in Pakistan but likely moving around far more frequently. A senior Pakistani diplomat told ABC News in late July that of course that's a possibility, but no one knows.

Bakos said she didn't have a guess as to where al-Zawahiri is now, but did not rule out north Africa, where al-Qaeda has a presence, especially since al-Zawahiri is originally from Egypt. Regardless, she said, the CIA is likely working around the clock to find him even as public attention and some government resources have shifted to the fight against ISIS.

"Just like bin Laden, it's an ongoing operation. You never give up on it," Bakos said.

A spokesperson for the CIA, which tracked bin Laden to a compound in Abbottabad, Pakistan, after an intense, decadelong manhunt, said the agency was aware of the new video but declined to comment further about it.

Though he has made relatively few public statements, al-Zawahiri, who took over al-Qaeda after U.S. forces killed bin Laden in May 2011, revealed himself last month in a series of videos in which he urged Muslims to unite against the "crusaders" in Muslim lands and also against the rival terrorist group ISIS. In the video released online today, al-Zawahiri is dressed similarly and sits before what appears to be the same curtain background as last month's videos.