A federal prosecutor in California said Friday he was investigating alleged fraud in this week's gubernatorial primary election, after President Donald Trump claimed that there was cheating.

Trump -- who cited fraud when he sought to overturn his 2020 presidential election loss -- has repeatedly cast doubt on the integrity of ballots in the liberal state.

On Thursday, the president complained  that there was still no definitive result two days after polls closed Tuesday.

First Assistant U.S. Attorney Bill Essayli -- who was appointed by Trump -- said Friday he would examine the way the vote was conducted.

"California's election system has serious structural vulnerabilities," Essayli wrote on social media.

"Universal vote-by-mail with no voter ID requirements creates conditions where fraud can go undetected and unpunished, eroding public confidence.

"My office has multiple election fraud investigations underway in coordination with (the FBI). We will follow the evidence wherever it leads and prosecute any violations of federal election law to the fullest extent."

With around two-thirds of votes counted, Trump-backed Republican Steve Hilton leads the tally in the state's gubernatorial primary, followed closely by Democrat Xavier Becerra, with another Democrat, Tom Steyer, a few percentage points behind.

The top two vote-getters go through to November's general election.Party is expected to lose significant ground.