WASHINGTON — Airline trade groups and even Transportation Secretary Sean Duffy have hit back at a proposal from Homeland Security Secretary Markwayne Mullin that would halt customs processing at “sanctuary city” airports refusing to cooperate with the Trump administration’s immigration agenda.

Mullin floated the plan to halt immigration procedures in an April Fox News interview shortly after being confirmed — and again in a May 13 meeting with airline and travel industry leaders, The Atlantic first reported.

Top travel trade associations have since come out publicly against the proposal, which was reportedly aimed at airports in New York City, Newark, Philadelphia, Chicago, Denver, Los Angeles, Seattle and San Francisco.

“Reducing CBP staffing at major airports would have a devastating effect on the airline and tourism industries, causing a significant operational disruption to carriers, travelers and the flow of international cargo,” Airlines for America, the largest trade association for the industry in the US, said in a statement.

The US Travel Association, which represents airlines as well as hotels and other travel businesses, said that for “domestic ⁠and international inbound travel alike, we urge governments at all levels to adopt policies that promote the ​free and efficient flow of legitimate travelers.”

Duffy himself batted down the suggestion in a House Budget Committee hearing on Thursday.

“We have people from around the world and around the country that need to be able to fly into all different kinds of places,” the cabinet official said. “We shouldn’t shut down air travel in a state that doesn’t agree with our politics.”

Around 50 million international travelers landed at the Big Apple’s three main airports alone in 2025.

In the April 6 Fox interview, Mullin had openly questioned whether sanctuary cities with international airports should “be processing customs.”

“Seriously, If they’re a sanctuary city, and they’re receiving international flights, and we’re asking them to partner with us at the airport but once [immigrants] walk out of the airport they’re not going to enforce immigration policy, maybe we need to have a really hard look at that because we need to focus on cities that want to work with us,” he told Fox News chief political anchor and “Special Report” host Bret Baier.

Asked whether cities would lose customs enforcement for international travelers, the DHS secretary responded: “We’re gonna have to start prioritizing things at some point.”

“Right now, remember, the Democrats are wanting to defund Customs and Border Patrol,” Mullin added.

President Trump has railed against sanctuary jurisdictions for refusing to coordinate with federal immigration enforcement — and threatened in January to pull all their funding.

Critics have argued that sanctuary policies prevent the deportation of violent criminals when prisons and jails ignore “detainer” requests from US Immigration and Customs Enforcement.

In August, the Department of Justice published a list of 12 states and 18 cities that are non-cooperative with such requests. Four of those cities — Chicago, Denver, Los Angeles and New York — are also home to four of the six busiest airports in America by 2025 self-reported passenger volume,

Reps for DHS declined to comment.