WASHINGTON — Kirstjen Nielsen, the homeland security secretary, said on Monday that cyberthreats against the United States were a national security crisis that she described as her top priority — not the situation for which President Trump last month declared a national emergency.

“On top of my list of threats, that many of you can guess, the word ‘cyber’ is circled, highlighted and underlined,” Ms. Nielsen said in a speech outlining her department’s focus in the coming year. “The cyberdomain is a target, a weapon and a threat vector all at the same time.”

Mr. Trump has called the increasing flow of immigrants to the southern border one of the most urgent national security issues threatening the United States. Last week, issuing his first veto against legislation that would have blocked him from diverting Defense Department funds to build a border wall, the president described a recent spike in migrants crossing the border as an “invasion.”

The Department of Homeland Security oversees customs officers and immigration agents who are on the front lines of the Trump administration’s campaign to close the border to illegal immigration.