The US Justice Department on Tuesday gave California officials 48 hours to review a list of alleged noncitizen voters or face legal action, according to a letter reviewed by Reuters - part of an unprecedented effort to find evidence of voter fraud ahead of next month's elections.

The letter, written by acting Deputy Attorney General R. Trent McCotter and addressed to California Secretary of State Shirley Weber, says the Justice Department provided officials in Los Angeles County with a "password-protected thumb drive" with the names of people the federal government "believes are likely noncitizens" and were sent mail-in ballots.

The letter demands that the state refuse to send ballots to those on the list until their citizenship status has been confirmed. Voters in question who have already received ballots should have their ballot "segregated" once it is mailed back; ballots already returned should not be tabulated "until citizenship has been determined," the letter stipulates.

The federal government "will have no choice but to seek emergency relief," according to the letter, if state officials do not follow its instructions. "Allowing noncitizens to vote cancels citizen votes," McCotter wrote.

The letter marks an escalation of the Trump administration's attempts to pressure states over alleged voting by non-US citizens and signals the Justice Department may be prepared to challenge ballots in the weeks leading up to the November 3 election, when Republicans will be defending their narrow majorities in Congress.

Justice Department spokeswoman Kiersten Pels confirmed the existence of the letter in a statement to Reuters, adding: "The department will take all necessary legal action to protect election integrity and ensure compliance with federal law.”Representatives from the White House and the California secretary of state's office did not immediately respond to requests for comment.

The Justice Department under President Donald Trump has repeatedly claimed that voting by non-US citizens is widespread, despite multiple studies showing it is rare.

Though states may have added more than 30,000 noncitizens to their voter rolls since 2000 due to software glitches and clerical errors, Reuters found in September, it was unclear how many had cast a ballot. Experts said the numbers were too small relative to the country’s 170 millioned voters to affect elections.

In July, the Justice Department sent letters to chief election officials in all 50 states warning they could face criminal prosecution if they knowingly allowed noncitizens to remain on their voter rolls or cast ballots.

The department's Civil Rights Division has demanded nonpublic voter data, including voters' Social Security numbers or dates of birth, from nearly all states as part of its hunt for noncitizens on state rolls. It has sued 30 Democratic- and Republican-led states that refused to hand over their rolls, but has lost 26 of those cases without winning one.

Reuters reported last month that the Department of Homeland Security was using a commercial data broker to find sensitive voter information not included in publicly available rolls, which officials and experts warned could lead to citizens being falsely flagged as illegal voters and potentially detained.

Federal prosecutors have also ramped up prosecutions of noncitizens for allegedly voting in past elections or submitting fraudulent registrations, touting 16 new criminal cases on Monday.

Officials in Democratic-led California have tussled for months with the Trump administration over its attempts to intervene in election procedures over allegations of voter fraud.

Homeland Security Secretary Markwayne Mullin informed California officials on July 16 that the federal government believed the state had noncitizens on its voting rolls, according to the letter.

California could prove pivotal in deciding control of the US House of Representatives, especially after the state's voters approved a new congressional map that heavily favors Democrats as part of a nationwide redistricting sprint that began in Republican-led states.

Voters in Los Angeles will also weigh in on a ballot measure that would allow certain noncitizen legal residents to cast ballots in municipal elections.