President Donald Trump announced that he would seek an investigation into alleged voter fraud after claiming that several million people voted illegally in November's election that brought him to power.
There is no public evidence of widespread illegal voting in last year's poll and Trump and the White House are yet to substantiate the president's claim.
"I will be asking for a major investigation into voter fraud, including those registered to vote in two states, those who are illegal and even those registered to vote who are dead - and many for a long time. Depending on the results we will strengthen up voting procedures!" Trump wrote on Twitter.
He did not elaborate beyond the two Twitter posts.
Hours after Trump told congressional leaders that as many as five million people could have voted illegally, White House spokesman Sean Spicer said that this belief was based "on the studies he's seen".
"I think the president has believed that for a while based on studies and information he has."
Trump lost the popular vote to Democrat Hillary Clinton by around 2.9 million votes, but won the all-important state-weighted electoral college.
Spicer said Trump had seen a study suggesting that 14 percent of people who voted "were non-citizens".
State officials in charge of the November 8 election have said that they found no evidence of widespread voter fraud and there is no history of it in US elections.
House of Representatives Speaker Paul Ryan, the most senior Republican in Congress, repeated on Tuesday that he had seen no evidence to support Trump's claims.
Trump has bristled at references to the popular vote results, which, combined with allegations of Russian tampering in the election on his behalf, has led to criticism that his victory was not legitimate.