CIA Director Mike Pompeo Michael (Mike) Richard PompeoUS reimposes UN sanctions on Iran amid increasing tensions Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Trump steps up Iran fight in final election stretch MORE said Monday the United States is resolute in its efforts against senior terrorist leaders, warning that their days are numbered.

"If I were them, I'd count my days," Pompeo told host Bret Baier on Fox News's "Special Report" after being asked if there are positive signs that the U.S. is close to catching top al Qaeda leaders such as Ayman al-Zawahiri or the son of Osama bin Laden.

"My team even as we sit here is working diligently to find them and bring them to justice," the former Kansas lawmaker said, saying it a mission they pursue daily.

On the anniversary of the Sept. 11, Pompeo claimed that the group behind New York's Twin Tower attack is "weaker than it was 16 years ago."

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"The U.S. government and our allies have done really good work to take out their senior leaders but the threat from radical Islamic terrorism remains and whether it is [the Islamic State of Iraq and Syria] or Ansar al-Sharia or any of the other couple of dozen groups that are out to do harm to America that threat remains, and we as a nation must remain ever vigilant so that something like what happened 16 years ago today can never happen again," he continued.

The CIA chief also stressed that ISIS has always "posed a serious threat" to the nation, pushing back on former President Obama's 2014 assessment that the group was like a "JV team" in terrorist circles.

"The U.S. estimate of ISIS has always been that they posed a serious threat. It's never been the case that this was a second-tier threat. In fact, there have never been as many foreign fighters in any one place as are in Syria even yet today — even in the heyday of al Qaeda there weren't as many foreign fighters. So, there's still a lot of work to do," Pompeo said.