President Donald Trump has promised a "major investigation into voter fraud", after making claims about millions of illegal ballots.
The new US president said he will be asking for a major investigation that would include "those registered to vote in two states, those who are illegal".
Mr Trump also said the probe would focus on "those registered to vote who are dead (and many for a long time)".
He has alleged that up to five million illegal votes were cast for Hillary Clinton, but has offered no proof.
"Depending on results, we will strengthen up voting procedures!" Mr Trump said of his planned inquiry.
Mr Trump is later expected to sign immigration-related decrees, including on his plan to build a wall along the US border with Mexico.
The president first made the claim about voter fraud in a late November tweet.
In addition to winning the Electoral College in a landslide, I won the popular vote if you deduct the millions of people who voted illegally— Donald J. Trump (@realDonaldTrump) November 27, 2016
Fact-checkers have rejected it as untrue and Republican election officials in key states have said they found no proof of fraudulent voting.
White House press secretary Sean Spicer said on Tuesday that Mr Trump continues to believe it, "based on studies and evidence that people have presented to him".
Mr Spicer was repeatedly pressed to specify such research, but failed to do so.
The unsubstantiated claim was started 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 fringe websites such as Infowars.com
Fact-checking website Snopes.com has said there is "zero evidence" that "illegal aliens" voted in the November election.
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.
Hillary Clinton, the Democratic candidate, received nearly three million votes more than Mr Trump, who won the presidency by prevailing in so-called swing states.
Republicans admonished Mr Trump and urged him to drop the matter. 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".
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".