WASHINGTON -- Department of Homeland Security Secretary John Kelly said he doesn’t know how to stop “homegrown terrorists,” despite saying that the homegrown threat is the “most common” threat facing the U.S.

“There are so many aspects to this terrorist thing,” Kelly said on CBS News’ “Face the Nation.” “Obviously you got the homegrown terrorists. I don’t know how to stop that. I don’t know how to detect that. You got other terrorist threats that come across the border.”

“I believe in the case of the murder -- in the Paris shooting I believe he was homegrown,” Kelly continued. “But, again, there are so many threats that come in from across border. And it’s essential absolutely to control one’s border.”

Kelly, a retired Marine Corps general, made the comments in response to a question about what the U.S. can learn from the deadly shooting on Paris’ Champs-Elysees last week for which the Islamic State of Iraq and Syria (ISIS) claimed responsibility.

When pressed on the administration’s ability to handle homegrown terrorism, Kelly reiterated that it is a “big threat.” He also again circled back to threats related to border security, a central tenet of President Donald Trump’s agenda.

“It is a big problem,” Kelly said. “It is -- you know, depending on where you sit is where you stand on this, It is a big threat. Is it the number one threat? I think it’s the most common threat. Unfortunately there are other similar-type terrorist threats that could come from outside the border. You know, the CIA., NSA, all the great men and women of DOD. are doing a great job keeping them away from the homeland.”

“The appeal I would make on the homegrown threat is if you see something, say something,” he said. “Whether you’re a parent, a sibling, an imam. And this extends frankly ... to white supremacists and that kind of terrorism as well. If you see a young man or a young woman going down that path where they’re always on these kind of websites or saying things at church or in a mosque that are clearly disturbing, then tell someone about it so that we can help that kid, young man or woman, before they break the law.”

On another national security front, Kelly said North Korea isn’t much of a threat right now “except in the world of cyber.”

“They’re pretty aggressive when they want to be in cyber,” he said.

Kelly also said that “the instant they get a missile that can reach the United States and they have a weaponized atomic device, a nuclear device on it, we are at grave risk as a nation.”