Attorney General Jeff Sessions fired a broadside at so-called "sanctuary cities" Monday, telling reporters local policies of noncooperation with immigration authorities are "dangerous" and will cost communities federal funding.

In the Trump administration's most pointed warning yet, Sessions said federal law allows withholding of federal funding to sanctuary cities, and signaled that such measures will soon be taken. Sessions, who took the podium at White House Press Secretary Sean Spicer's regular media briefing, warned of a pending crackdown by the administration.

"Such policies cannot continue," he said. "They make our nation less safe by putting dangerous criminals back on the street."

"Such policies cannot continue. They make our nation less safe by putting dangerous criminals back on the street." — Attorney General Jeff Sessions

While not a technical term, "sanctuary cities" are communities that have refused to work with Immigration and Customs Enforcement officials after detaining illegal immigrants. By federal law, they are required to inform the feds when they have an illegal immigrant in custody, even if he or she has not been convicted of a crime.

Several big cities, including New York, Chicago and Los Angeles, as well as dozens and possibly hundreds of smaller counties, cities and towns, also refuse to notify ICE, which can then come and take custody of the illegal immigrant, possibly for deportation.

“LAPD has never participated in programs that deputize local law enforcement to act as immigration agents, and on my watch they never will,” Los Angeles Mayor Eric Garcetti said last week.

A spokesman for Chicago Mayor Rahm Emanuel said the administration's plan to hold back federal funds was "no different than what was in the Executive Order [travel ban] the president signed weeks ago."

"The administration's plan to deny federal funds to cities that are standing up for their values is unconstitutional," said Matt McGrath, a spokesman for Chicago mayor Rahm Emanuel. "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."

Immediately after Sessions spoke, New York State Attorney General Eric Schneiderman, a staunch critic of Trump, said he will fight any efforts to defund sanctuary communities in the Empire State.

“My office will continue to ensure local governments have the tools they need to legally protect their immigrant communities – and we won’t stop fighting to beat back President Trump’s un-American immigration policies,” Schneiderman said in a statement.

But Sessions said such policies put citizens' safety in jeopardy.

"The American people know that when cities and states refuse to help enforce immigration laws, our nation is less safe," Sessions said.

Perhaps telegraphing action President Trump warned of during his campaign, Sessions said the administration will pull billions in federal funding to sanctuary communities if they remain in noncompliance.

Sessions said communities applying for Department of Justice grants will be required to show they are following immigration law.

The DOJ will withhold, and could potentially "claw back" grants to localities out of compliance with federal immigration law, Sessions said. He noted one Justice Department office alone was expecting to award more than $4.1 billion in grants this fiscal year.

"Failure to deport aliens who are convicted of criminal offenses puts whole communities at risk, especially immigrant communities in the very sanctuary jurisdictions that seek to protect the perpetrators," Sessions said.

Sessions, an early supporter of Trump's candidacy, is a longtime illegal immigration hawk who helped drive Trump's winning platform plank on the subject.

Early in Trump's candidacy, in July 2015, a woman named Kate Steinle was killed in San Francisco by an illegal immigrant who had been deported previously and had recently been freed by local authorities. The murder became a rallying point for the campaign.

More recently, a 14-year-old Maryland high school girl was raped in a school bathroom allegedly by two men, at least one of whom is an illegal immigrant. That case has reignited the debate about illegal immigration and sanctuary policies.

Just days after his inauguration, Trump ordered the Department of Homeland Security to publish a weekly list of all detainer requests turned down by local jails. Trump said the list will "better inform the public regarding the public safety threats associated with sanctuary jurisdictions."