President Trump said in an interview posted online Tuesday that he planned to designate Mexican drug cartels as terrorist organizations, owing to what he said was the high number of Americans killed by their activities.

The comments, which were made in an interview with the former Fox News host Bill O’Reilly and posted to his personal website, represent a shift in United States policy that the Mexican foreign minister, Marcelo Ebrard, told reporters on Monday he did not believe would happen.

“I will be designating the cartels,” Mr. Trump said in the interview. “I have been working on that for the last 90 days. You know, designation is not that easy, you have to go through a process, and we are well into that process.”

Deadly violence by drug cartels in Mexico gained new attention in the United States in recent weeks after the killing of six children and their three mothers, all dual Mexican and American citizens, who were part of a fundamentalist Mormon community in the north of the country.