Secretary of Homeland Security John Kelly shot back at former President Obama’s comments deriding the construction of a border wall Friday.

“We’re not hiding behind a wall. We’re constructing physical barriers backed up by technology, and of course the great men and women of customs and border protection, to simply safeguard our Southern border,” he said on Fox News. “We’re not hiding behind a wall, and you can’t defend anything by hiding behind something.”

Former President Obama told a crowd in Berlin that “in this new world we live in, we can’t isolate ourselves. We can’t hide behind a wall.” In the thinly veiled swipe against President Trump, who’s vowed to construct a border wall between the U.S. and Mexico, Obama added that people should “fight against those who divide us.”