A federal appellate panel said on Friday that Iowa could not enforce a Republican-backed law that made it a state crime for some undocumented immigrants to enter the state. The ruling keeps in place a lower court’s injunction that blocks, at least for now, Iowa’s attempt to change how immigration crimes are policed in the United States.

The fight in Iowa comes as part of a broader effort by conservative states to carve out a role in immigration enforcement by creating their own laws. In doing so, they entered legal territory that former President Joseph R. Biden Jr.’s Justice Department argued should be the exclusive domain of federal officials.

President Trump is taking a much harsher approach to illegal immigration in his second term, and has promised mass deportations. But it was not yet clear how his Justice Department would approach cases like the one in Iowa. Justice Department officials did not immediately respond to a request for comment on Friday.

The Iowa attorney general’s office said it was evaluating its next steps in the case. Attorney General Brenna Bird, a Republican, said in a statement that “despite today’s court ruling, the battle is far from over.”

“As President Trump works nationally to fix the mess Biden and Harris created on the southern border,” Ms. Bird added, “we will continue fighting in Iowa to defend our laws and keep families safe.”

The three-judge panel of the U.S. Court of Appeals for the Eighth Circuit that heard the Iowa case was composed entirely of jurists appointed by Republican presidents. It included one judge appointed by Mr. Trump, one by former President George W. Bush and one by former President George H.W. Bush.