President Donald Trump believes that millions of people voted illegally in the US election based on "studies and evidence", the White House has said.
Press secretary Sean Spicer said Mr Trump "does believe that", but offered no evidence to support the claim when pressed by reporters.
Mr Trump has repeated his claim to explain why he lost the popular vote to Democrat Hillary Clinton.
But any notion of widespread voter fraud has been widely rejected.
"He continues to maintain that belief based on studies and evidence that people have presented to him," Mr Spicer told reporters on Tuesday.
Mr Spicer's comments came after the president told congressional leaders behind closed doors on Monday night that three to five million undocumented immigrants had illegally voted in the election.
Mr Trump, who first made the claim in a late November tweet, has never provided any evidence.
Fact-checkers have rejected it as untrue and Republican election officials in key states have said they found no proof of fraudulent voting.
On Tuesday, the National Association of Secretaries of State said it had confidence in the "systemic integrity of our election process" and was not aware of any evidence related to Mr Trump's claims.
The Democratic candidate, Hillary Clinton, received nearly three million votes more than Mr Trump, who won the presidency by prevailing in so-called swing states.
Is there any evidence of illegal votes?
- Unsubstantiated claim was started without evidence by self-styled conservative voter fraud specialist Greg Phillips, who tweeted "Number of non-citizen votes exceeds 3 million"
- His tweets were picked up by right-wing websites like Infowars.com, which has made false claims in the past
- Fact-checking website Snopes.com says there is "zero evidence" that "illegal aliens" voted in election
- 'Don't buy it', says Politifact, which points to research suggesting there have been 56 cases of non-citizens voting numbers between 2000-2011
- A Pew study in 2012 found millions of invalid voter registrations because people moved or died, but "zero evidence" of fraud
Republicans admonished Mr Trump and urged him to drop the matter a day after the closed doors meeting with congressional leaders.
Senator Lindsey Graham called the comments "inappropriate", adding that Mr Trump should "knock this off".
He continued that the president "seems to be obsessed with the idea that he could not have possibly lost the popular vote without cheating and fraud".
Trump must move on - Anthony Zurcher, BBC News, Washington
Donald Trump just can't let it go. Even with a comfortable electoral college victory over Hillary Clinton last November, his three-million-vote deficit to his Democratic presidential opponent clearly is eating at him. It's an open wound on his psyche that leaves him looking for someone to blame.
How else to explain the president's continued allegations that undocumented immigrants cast millions of illegal votes for his opponent, despite no supporting evidence?
It would be in Mr Trump's interest to simply move on. Or, if he truly feels his concerns are legitimate, to announce a robust effort to investigate what would easily be the biggest electoral heist in US history.
Instead, he has vented his angst in early morning tweets and during closed-door meetings with his members of Congress. The end result is a distracting media frenzy that forces his Republican allies - including Speaker of the House Paul Ryan - to distance themselves from his unsubstantiated accusations.
It's creating a rift within conservative ranks when the one thing Mr Trump needs - in fact, the only thing he really needs to enact his agenda - is party unity.
House Speaker Paul Ryan also said there was no evidence to support his claims.
Republican Pennsylvania Representative Charlie Dent also weighed in, saying Mr Trump needed to move on and "get to the serious business of governing".
Independent Senator Bernie Sanders said it was "nonsensical" and he feared Mr Trump was paving the way for Republican governors to "go forward with voter suppression".