WASHINGTON — Attorney General Jeff Sessions on Monday warned so-called sanctuary cities they could lose federal money for refusing to cooperate with immigration authorities and suggested the government would come after grant money that has already been awarded if they don't comply.

Sessions said the Justice Department would require cities seeking some of $4.1 billion available in grant money to verify that they are in compliance with a section of federal law that allows information sharing with immigration officials.

His statements in the White House briefing room echoed President Donald Trump's nationalist campaign rhetoric and came just three days after the administration's crushing health care defeat.

But Sessions also said he was clarifying a similar policy adopted by the Obama administration last year.

"I urge the nation's states and cities to carefully consider the harm they are doing to their citizens by refusing to enforce our immigration laws," Sessions said.

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Trump had said during the campaign that he would "defund" sanctuary cities by taking away their federal funding. But legal precedent suggests that would be difficult.

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Sessions' message came days after the administration released a report on local jails that listed more than 200 cases of immigrants released from custody before federal agents could intervene. That list was compiled following an executive order Trump signed in January that called on the government to document which local jurisdictions aren't cooperating with federal efforts to find and deport immigrants in the country illegally.

Meanwhile, municipal leaders gathered in New York vowed to defy Trump's crackdown as they gathered for a small conference that attracted officials from cities including San Francisco, Seattle, Denver, Chicago, Philadelphia and New York.

"We are going to become this administration's worst nightmare," said New York City council speaker Melissa Mark-Viverito.

Related: Conservative Cities See 'Sanctuary City' Term as Scarlet Letter

Mark-Viverito and others promised to block federal immigration agents from accessing certain private areas on city property, to restrict their access to schools and school records and to offer legal services to immigrants in the country illegally.

City officials were also encouraged to embrace their rarely used oversight and subpoena powers to investigate federal immigration practices.

Mayors across the country put out statements following Sessions comments:

“President Trump’s latest threat changes nothing," said New York Mayor Bill DeBlasio. "We will remain a city welcoming of immigrants who have helped make our city the safest big city in the nation. Any attempt to cut NYPD funding for the nation’s top terror target will be aggressively fought in court. We won’t back down from protecting New Yorkers from terror — or from an overzealous administration fixated on xenophobia and needless division.”

"The safety and well-being of our residents is, and will continue to be, my top priority as Mayor of Boston," Boston Mayor Martin Walsh said it a statement. "The threat of cutting federal funding from cities across the country that aim to foster trusting relationships between their law enforcement and the immigrant community is irresponsible and destructive."

“The Attorney General's comments this morning are no different than what was in the Executive Order the president signed weeks ago," said Matt McGrath, a spokesman for Chicago Mayor Rahm Emanuel. "The administration's plan to deny federal funds to cities that are standing up for their values is unconstitutional, and Chicago is proud to stand with 34 cities and counties across the country in asking a federal court to prevent the federal government from illegally withholding federal funds.”

Related: Trump’s Immigration Orders Could Drive Crime Up, Experts Say

“The Justice Department’s leadership has once again shown blind allegiance to ideology rather than a sound understanding of public policy," said Syracuse Mayor Stephanie A. Miner. "Cities, like Syracuse, are sanctuary cities because it is in keeping with our history of welcoming immigrants and refugees. It is also a good policy because it enables our law enforcement agencies to build relationships with the community, so those who are victim to or witness crime are able to come forward without fear of retaliation."

She added, "This divisive decision is against our history as cities and as a nation. This is where the Statue of Liberty watches guard in New York Harbor and proudly proclaims the United States accepts the huddled masses yearning to breathe free. We are the nation where Alexander Hamilton started as a clerk and changed our history and where thousands of New Americans are breathing life into Syracuse’s neighborhoods and contribute mightily to our local economy. This is not who we are and this is not who we should be."