NEW YORK (Reuters) - The Department of Justice is reviewing letters from 10 local jurisdictions that said they are in compliance with U.S. immigration law, to determine whether to cut federal funding, officials said Thursday, heating up a dispute between so-called sanctuary cities and President Donald Trump’s administration.

Immigrant supporters protest during the Los Angeles City Council ad hoc committee on immigration meeting in Los Angeles, California, U.S., March 30, 2017. REUTERS/Lucy Nicholson

In April, the department had asked a handful of states and cities to document by June 30 their compliance with a statute that says local governments cannot prevent their employees from sharing information with U.S. immigration officials.

The Trump administration has said jurisdictions that do not fully cooperate are shielding “criminal illegal aliens,” and has promised to crack down on cities that do not comply. The sanctuary jurisdictions say they are following the law and do not want to spend local resources on immigration enforcement.

“It is not enough to assert compliance, the jurisdictions must actually be in compliance,” U.S. Attorney General Jeff Sessions said in a statement on Thursday. He said the 10 jurisdictions had written in with “alleged compliance information” and that the government would “examine these claims carefully.”

Sessions’ statement said “some of these jurisdictions have boldly asserted they will not comply with requests from federal immigration authorities.” If the government finds the cities are violating the statute, known as Section 1373, it could decide to cut federal funds.

In the letters seen by Reuters, the jurisdictions say they are following the law even though some do not honor all “detainer” requests sent by Immigration and Customs Enforcement (ICE.) A “detainer” asks local authorities to hold people in jail up to 48 hours beyond when they are set to be released so immigration officials can take them into custody.

Many of the letters noted that compliance with detainer requests is voluntary and is not required under the statute. The jurisdictions targeted are the states of California and Connecticut, Chicago and Cook County in Illinois, New Orleans, Philadelphia, Las Vegas, Miami, Milwaukee and New York.

At least one of the jurisdictions – Nevada’s Clark County, which is dominated by Las Vegas – has a long-standing formal agreement with ICE in which local police officers help with federal immigration enforcement.

New York City said it complies with detainer requests for people who have been convicted of certain “violent or serious” crimes, so long as the request is accompanied by a judicial warrant. Like other cities, New York said its priority is creating trust between immigrant communities and local police to encourage residents, even if they are living in the country illegally, to report crimes.

Mitchell Landrieu, the Mayor of New Orleans made a similar argument in a letter to Sessions. He said the administration has erroneously characterized sanctuary cities as havens for Central American gangs. Landrieu said an audit of gangs in New Orleans did not find a single Latino-dominated group.

“Undocumented people who commit violent crimes must face the criminal and immigration legal systems of this country. But that does not mean that all people are illegal immigrants that are part of violent gangs,” Landrieu wrote.

Milwaukee County Executive Chris Abele called the Justice Department’s statement on Thursday “inflammatory.”

The county is at risk of losing more than $6 million in revenue if the Justice Department follows through, a June 28 letter from its lawyers said. It said Milwaukee would “avail itself of all legal options available” to “protect its grant funding.”

Trump’s executive order early in his presidency pledging to cut funding to sanctuary cities has been challenged in the courts. In April, a federal judge in San Francisco said in a case brought by Santa Clara county that cities were likely to succeed in proving Trump’s order unconstitutional.

The California county wrote in a court filing on Thursday that top administration officials have repeatedly stated that federal funding should be tied to local willingness to honor ICE detainer requests.