LOS ANGELES — The federal government's effort to flag noncitizens on California's voter rolls has run into a direct challenge from the county that holds more registered voters than any other in the state.

Election officials in Los Angeles County say a preliminary analysis of the Department of Justice's list of 20,000 people described as “likely” noncitizens registered to vote in California shows the list is “unreliable,” NPR reported early Wednesday. Some of the people named on the list appear to be U.S.-born citizens, according to the county's review.

The list has been at the centre of a running legal and political fight between Washington and Sacramento over who is on California's rolls and how citizenship is verified, with the Justice Department pressing state officials over ballot eligibility. County officials, who actually maintain the rolls, are now questioning the quality of the federal data itself.

The dispute lands weeks before Americans vote in the midterm elections, and it raises the stakes for any voter wrongly flagged: being labelled a likely noncitizen can trigger scrutiny of a registration that may be entirely lawful. California officials have not said how many names on the list the county has checked so far.