Homeland Security Secretary John Kelly defended the plan for a wall on the border between the U.S. and Mexico, while rejecting a claim made by former President Barack Obama.

"We're not hiding behind a wall. We're constructing a physical barrier backed up by technology and, of course, the great men and women of the Customs and Border Protection to simply safeguard our southern border," Kelly told Fox News' "Fox & Friends" program on Friday.

"It's not going to impede in any way the legal crossing of millions of people a week, Mexicans and all other nationalities, Americans going south and north. Normal commercial traffic will move. You can't defend anything by hiding behind something."

Kelly was responding to a Thursday statement by Obama, where he said, "In this new world that we live in, we can't isolate ourselves. We can't hide behind a wall."

During an appearance on "CBS This Morning" on Friday, Kelly was asked about whether the U.S. should anticipate an attack like what happened in Manchester.

"The reality is they're attempting everything all the time," Kelly told host Charlie Rose. "The very, very good news, is that we have tremendous professionals fighting the away game for us."

Information from ordinary Americans is vital, Kelly continued, and "every citizen, in my view, is an intelligence collector, not in the sense of watching people too closely. But if you see something, say something."