US President Donald Trump has signed an executive order that aims to overhaul US federal elections, including by requiring voters to show proof of citizenship and limiting when states can receive mail-in ballots.

Experts warn the move could disenfranchise millions of Americans who do not have easy access to a passport or other legal documents proving they have the right to vote.

It is unclear how enforceable the order is, given US states have wide legal leeway to determine how they run their elections. It is expected to be challenged in court.

The order, titled "Preserving and Protecting the Integrity of American Elections", was signed by Trump on Tuesday at the White House.

"Election fraud. You've heard the term. We're going to end it, hopefully. At least this will go a long way toward ending it," Trump said as he signed the order on Tuesday.

The order says that the US has failed "to enforce basic and necessary election protections" and calls on states to co-operate with the the White House or risk losing access to federal funding if they do not require proof of citizenship.

It is already illegal for non-citizens to vote in elections.

The Illegal Immigration Reform and Immigrant Responsibility Act of 1996 prohibits non-citizens from voting in federal elections.

Every state is required to use a common registration form that requires people to confirm they are US citizens, under penalty of perjury for false claims, but does not require documentary proof.

Experts say there have been very few cases of immigrants voting illegally in US elections.

The order also seeks to bar states from accepting postal ballots received after election day. Currently, 18 states allow ballots to be received after election day as long as they were mailed on or before the day of the vote.

The order would withdraw federal funding for US states that do not comply.

Trump has been accused of spreading election misinformation, including by claiming that "millions" of illegal immigrants voted in his first election campaign. He also continues to deny that he lost the 2020 election to Joe Biden.

Previous efforts to pass a voter ID law in Congress have failed.

Democrats who have criticised similar past reform attempts have pointed to statistics showing that a large number of Americans do not have an enhanced drivers licence or passport for ID.

The legal basis for the order is expected to be challenged in court.

"The president cannot override a statute passed by Congress that says what is required to register to vote on the federal voter registration form," Wendy Weiser, from the Brennan Center for Justice at New York University, told the Washington Post.

UCLA law professor Rick Hasen said on his blog that elections are largely run by each individual state government, and that if allowed to stand, the order would radically shift power to the federal government.