WASHINGTON, May 28 (Reuters) - The head of the U.S. Homeland Security Department said Thursday the Trump administration could soon stop processing international travelers ‌and cargo at Newark airport because local law enforcement are not assisting federal immigration officials in northern New Jersey.

"If things don't change, we're going to have to make this step pretty quick," Secretary Markwayne ​Mullin told Fox News' "Fox and Friends" of the major United Airlines UAL.O hub near New York City.

Mullin complained that local police were not ensuring that federal immigration officials could enter and exit a New Jersey detention center and warned he could reassign customs officials from the airport. "If (Customs) isn't there processing international flights, then those individuals when the airlines land won't be permitted into the United States," Mullin said.

On Tuesday, ​Mullin said the Trump administration was drawing up plans to stop processing international travelers ‌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 next month's start of the FIFA World Cup.

The final will be held July 19 in East Rutherford, New Jersey, about 12 miles from Newark airport.

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

The U.S. Justice Department published a ​list of so-called sanctuary cities and states that included many cities with major ​international airports including Boston, Denver, Philadelphia, Chicago, Los Angeles, New York City, Newark, Seattle and San Francisco.

Mullin first publicly made the threat in April.

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

The U.S. Travel Association said halting international travel at major airports would have devastating consequences for the travel industry and communities dependent on international visitors.

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