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.

After that ambush, Mr. Trump said on Twitter that the time had come “for Mexico, with the help of the United States, to wage WAR on the drug cartels and wipe them off the face of the earth.”

In the interview posted online Tuesday, Mr. Trump declined to say what measures he would consider taking once the cartels had been designated as terrorist organizations. When Mr. O’Reilly asked if drone strikes in Mexico were a possibility, the president replied, “I don’t want to say what I am going to do, but they will be designated.”

“Look, we are losing 100,000 people a year to what is happening and what is coming through from on Mexico,” the president said, without elaborating. “They have unlimited money, the people, the cartels, because they have a lot of money, because it is drug money and human trafficking money.”

The administration of President Andrés Manuel López Obrador of Mexico seemed to be caught off-guard by Mr. Trump’s declarations.

In a statement issued Tuesday night, the Mexican Foreign Ministry said it had “entered into communication with the various corresponding authorities” of the United States “to know the content and the reach” of Mr. Trump’s statements. The ministry also said it would seek to set up a high-level meeting “as soon as possible” to address the issue.

“The Government of Mexico will continue its diplomatic work towards an understanding that, through cooperation and intelligence, guarantees the security of both countries,” the statement said.

Mr. Ebrard, the foreign minister, had said a day earlier that he did not expect the United States government to seek to designate Mexican drug trafficking organizations as terrorists, and suggested that the two countries were already working together closely enough in the fight against those groups without the need for that legal designation.

“You do not need to designate or classify a specific group as terrorist so that we act together against it,” Mr. Ebrard said.

Once a group has been designated as a terrorist organization, it is illegal for people in the United States to knowingly provide support for it, and its members are barred from entering the country. Financial institutions are also forbidden from doing business with the organization.

Arturo Sarukhán, a former Mexican ambassador to Washington, said that the designation would create “a series of collateral effects” with profound implications for the relationship between the United States and Mexico.

The designation would not only have an impact on the bilateral strategy to combat narcotrafficking, but could also have “very broad and brutal political, diplomatic, financial, economic and commercial consequences,” he said in an interview on Milenio Televisión, a Mexican network.

It would also have the political effect of reinforcing “this narrative of Mexico as a threat to the national security of the United States,” Mr. Sarukhán said, noting that the timing coincided with the start of the presidential campaign.

“It’s something that really can be very, very, very disruptive for the bilateral relationship,” he said.

The interview between Mr. Trump and Mr. O’Reilly that was posted online on Tuesday evening was a clip from a longer conversation between the two. Mr. O’Reilly said that the full interview would be available to people who bought a premium membership to his personal website.