Washington DC - The Democratic mayors of four US cities were hauled over the coals in Congress Wednesday over so-called "pro-criminal" immigration policies, as Republicans seek to back up President Donald Trump's aggressive mass deportation effort.

Boston's Michelle Wu, Chicago's Brandon Johnson, Denver's Michael Johnston, and Eric Adams of New York were dressed down over "sanctuary city" policies that limit cooperation with federal immigration agents.

All four cities have struggled to cope with large numbers of migrants crossing from Mexico, and lawmakers in the Republican-led House Oversight Committee pressed the leaders on crime in their streets.

"Sanctuary cities make us all less safe and are a public safety nightmare. That is why I led an investigation into sanctuary cities and why these mayors are here today," said committee chairman James Comer.

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"We cannot let pro-criminal alien policies and obstructionist sanctuary cities continue to endanger American communities... Today, Mayors Wu, Johnson, Johnston, and Adams will be publicly accountable for their failure to follow the law and protect the American people."

Adams of New York was expected to avoid the worst barbs as he made a show of cooperating with the White House on immigration, after Trump's Justice Department dropped a criminal corruption and bribery case against him in an alleged quid-pro-quo.

Adams, who is running for another term, denies that his strong line on immigration is connected to the Justice Department's decision, claiming that he is "just doing what he thinks is right."

The former police officer defended New York's sanctuary status, however, stating that crime was down last year, and that the city had just seen three straight months of double-digit declines in major offenses.

"To be clear: A 'sanctuary city' classification does not mean our city will ever be a safe haven for violent criminals. It also does not give New York City the authority to violate federal immigration laws," he said.

"To the contrary, New York City will always comply with city, state, and federal laws, as it does now. Law-abiding immigrants in New York have an important role."

Although the policies vary depending on the location, sanctuary cities typically prohibit public officials telling federal agents about undocumented immigrants at risk of deportation.

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Denver's Johnston said that after he was sworn into office 20 months ago, he was seeing up to 300 people a day arriving in buses and in danger of freezing to death on the streets.

Some 42,000 people arrived over 18 months in what he termed "the largest per capita influx of any city in America," and he detailed how the city had helped them with housing, job training, and other programs.

"Denver made a choice as a city, not to hate each other, but to help each other, not to turn on each other, but to turn to each other and see if together we could solve a problem that felt bigger than any one of us," he said.

"And that's what we did. It wasn't perfect, and it required sacrifice from all of us, but in the end, Denver came out stronger and closer than we were before."

The hearing comes with Vice President JD Vance visiting the US-Mexico border at Eagle Pass in Texas to tour an immigration processing facility.

Vance, the highest-ranking member of Trump's second administration to visit the border, is expected to be joined by Defense Secretary Pete Hegseth and Tulsi Gabbard, Trump's director of national intelligence.