U.S. Homeland Security Secretary Markwayne Mullin said the Trump administration is drawing up plans to stop processing international travellers and cargo at ​major U.S. airports in "sanctuary cities" that have declined to cooperate with an immigration crackdown.

The move could effectively halt international air travel and commerce at major airports in Democratic states, with millions of foreign tourists expected to stream in for the start of next month's FIFA World Cup.

Mullin told Fox News Channel's Sean Hannity in an interview broadcast on Tuesday that he had met White House officials but emphasised that no decision had been made on whether to proceed.

"We are currently - ⁠which we're not initiating yet - but we're currently drawing up plans," Mullin said.

"We shouldn't be processing ​international flights into their cities," he added, in a reference to sanctuary cities, where he suggested, "local ​radical left Democrats aren't allowing us to do our job and enforce federal laws."

Last week, Reuters and other media reported that Mullin had privately told U.S. travel executives his department could opt to stop customs and immigration processing ​of international travellers.

The U.S. Justice Department published a list of so-called sanctuary cities and states that ​included many cities with major international airports.

Among these were Boston, Denver, Philadelphia, Chicago, Los Angeles, New York City, Newark, ‌Seattle ⁠and San Francisco.

Mullin first publicly made the threat in April during a dispute over funding for his department, but said on Tuesday the idea was under active consideration.

Democrats say reforms are needed to rein in abuses by Immigration and Customs Enforcement and Customs and Border Protection (CBP), including the killing of ​two U.S. citizens in ​Minneapolis in January.

The U.S. ⁠Travel Association, which represents major airlines, hotels, car rental firms and other travel companies, told Reuters on Friday its representatives had met Mullin.

Mullin "confirmed his ​previous comments that the administration is considering a withdrawal of CBP officers ​from international airports ⁠in certain sanctuary cities," U.S. Travel added in a statement to Reuters.

It also flagged the devastating consequences for the travel industry and communities dependent on international visitors.

More than 50 million international travellers arrived at ⁠the ​three major New York airports alone last year.

Reducing customs staffing at major airports would significantly disrupt operations for carriers, travellers, and the flow of international cargo, Airlines for America, a grouping of major passenger and cargo airlines, said last week.