Former President Donald Trump's plan to deport as many as 20 million migrants from the U.S. would likely involve "large operations" in so-called sanctuary cities that don't cooperate with federal immigration officials, according to a report Sunday.

Two former Immigration and Customs Enforcement officials appointed by Trump told the New York Post that ICE has "systems in place that are very good at identifying people" and that immigration officers would probably be temporarily relocated to round them up.

"This effort will likely include citywide operations where officers from different parts of the country are brought in to conduct work site investigations and make arrests within the course of weeks within in any given jurisdiction," former ICE chief of staff Jon Feere said.

Feere said, "ICE would prefer that all states and cities cooperate with federal law enforcement."

"But those that choose not to are going to see an increase in operations within their communities. ICE will have no choice but to conduct large operations," he said.

Major cities that have declared themselves migrant sanctuaries include New York City, Los Angeles, Chicago, and Philadelphia, according to the Center for Immigration Studies, which advocates for reduced immigration to the U.S.

Former ICE acting director Tom Homan said the agency "would still prioritize criminals and national security threats first, they are the most dangerous for the country."

"But I would say no one is off the table. If you're in this country illegally...then we'll remove you," he said.

Homan said much of Trump's plan, which the presumptive Republican presidential nominee discussed during a recent interview with Time magazine, would be "up to Congress" because of the spending it would require.

"We need officers, we need detention beds, we need transportation contracts," Homan said.

Feere, however, said ICE already had the capacity to detain more migrants than those being held now.

ICE had 34,373 migrants in detention as of April 21, according to Syracuse University's Transactional Records Access Clearinghouse.

During an interview for a report published on April 30, Trump told Time that, if elected, he would use local law enforcement and the National Guard to launch the largest mass deportation operation in American history.

When asked if he would also use "the military inland as well as at the border," Trump said, "I don't think I'd have to do that."

"I think the National Guard would be able to do that. If they weren't able to, then I'd use the military," he said.

Trump also said he didn't envision building new migrant detention camps before saying, "I would not rule out anything."

"But there wouldn't be that much of a need for them, because of the fact that we're going to be moving them out. We're going to bring them back from where they came," he said.