President Donald Trump on Thursday celebrated what he called "a military operation" to round up and deport undocumented immigrants who have committed crimes or caused violence in the United States.

"We're getting gang members out, we're getting drug lords out, we're getting really bad dudes out of this country - and at a rate that nobody's ever seen before," Trump told a group of several dozen manufacturing executives during a policy discussion session at the White House.

Trump talked about the trip Secretary of State Rex Tillerson and Homeland Security Secretary John Kelly are making to Mexico this week. Trump said he told Tillerson, "That's going to be a tough trip because we have to be treated fairly by Mexico."

Trump congratulated Kelly for the work his department is doing to secure the border with Mexico and deport illegal immigrants.

"It's a military operation," Trump said, attributing gang violence and illegal drug trade to illegal immigrants.

Trump's reference to a military operation could raise eyebrows among immigrant rights advocates and even within the Department of Homeland Security. Federal immigration policy is enforced by several divisions inside DHS, including Customs and Border Patrol and Immigration and Customs Enforcement, and the military has no role. A leaked DHS proposal last week to deploy Army National Guard troops to help apprehend undocumented immigrants was quickly denied by DHS leadership and the White House as under consideration. The unions representing border patrol agents and officers have regularly denounced the use of military personnel.

"I do not believe the National Guard to be a good idea," Brandon Judd, president of the National Border Patrol Council, said in an interview. "We're just setting ourselves up for too much liability with people who have not been trained to do the jobs."

Trump was presumably referring to a series of enforcement actions carried out by ICE two weeks ago that rounded up 683 immigrants purportedly in the country illegally.

Similar raids were carried out during the Obama administration and the department downplayed the significance of the number in a Feb. 13 news release.

"ICE conducts these kind of targeted enforcement operations regularly and has for many years," DHS said.

But immigrant rights advocates have said the directives contained in a pair of memos from Kelly this week would dramatically expand the pool of immigrants who would be targeted for deportations, broadening it well beyond the hardened criminals and new arrivals that had been the priorities under the Obama administration.

On Thursday, Trump portrayed the immigrants targeted as criminals.

"They're rough and they're tough, but they're not tough like our people, so we're getting them out," Trump said.

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David Nakamura contributed to this story.