WASHINGTON, United States (AFP)—A US appeals court on Thursday suspended the release of hundreds of people on bail after they had been arrested by immigration authorities in the midwestern state of Illinois as part of President Donald Trump’s crackdown.

In September, the Trump administration began an operation by Immigration and Customs Enforcement (ICE) in Illinois, dubbed “Midway Blitz,” targeting suspected undocumented immigrants, particularly in its major city, Chicago, a stronghold for Democrats.

Last week, a federal judge in Chicago ruled in favor of lawyers representing more than 600 people who had been detained by ICE, challenging the legality of their arrests by saying there were no reasonable grounds for detention or outstanding warrants.

The ruling said the judge would order the release of about 450 of the detainees on Friday after they each pay $1,500 in bail and agree to surveillance measures, such as ankle monitors.

However, the appeals court — after a filing by the Trump administration challenged the ruling — suspended the release of the detainees until it decides on the matter, setting a hearing for December 2.

Since returning to the White House in January, Trump has made cracking down on illegal immigration a top priority, often referring to it as an “invasion.”