A fake robocall claiming to be from US President Joe Biden has urged voters to skip Tuesday's primary election in New Hampshire.

An investigation has been opened into the calls, which are suspected to have been artificially generated.

It is unclear who created the audio. The Trump campaign has denied involvement.

In the message, a voice tells voters to "save" their ballot for the general election in November.

"Republicans have been trying to push nonpartisan and Democratic voters to participate in their primary. What a bunch of malarkey," says the message.

But there is no rule that says people who vote in a primary cannot cast a ballot in the general election, so there is no reason to "save" a vote.

The New Hampshire attorney general's office said in a statement: "Although the voice in the robocall sounds like the voice of President Biden, this message appears to be artificially generated based on initial indications."

It added that the "messages appear to be an unlawful attempt to disrupt" the election on Tuesday, and that "voters should disregard the content of this message entirely".

To recipients, the robocalls falsely appeared to be coming from the personal mobile phone number of a former state Democratic chairwoman, Kathy Sullivan, who helps run a pro-Biden political group in New Hampshire.

"This call links back to my personal cell phone number without my permission," she said in a statement, calling it "outright election interference".

Anti-robocall application Nomorobo estimates the calls were placed between 5,000 and 25,000 times, reports CNN.

On Monday, White House spokeswoman Karine Jean-Pierre confirmed that the audio was fake. Biden campaign manager Julie Chavez Rodriguez criticised the call as an attempt to "suppress voting and deliberately undermine free and fair elections".

In the New Hampshire primary, voters from both main parties are picking which candidate should be their standard-bearer in this year's general election.

Though Mr Biden's name will not appear on the New Hampshire primary ballot, a campaign led by state and local Democrats has taken shape for voters to write in his name.

The decision to remove his name came after Mr Biden approved a new voting calendar that stripped New Hampshire of its status as the first state to hold a primary.

Mr Biden is all but certain to win the state and his party's nomination to seek a second term in the White House.