WASHINGTON - US President Donald Trump said Wednesday that he will order a probe into alleged voter fraud in the presidential election that brought him to power.

"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 results, we will strengthen up voting procedures!" Trump said in two consecutive tweets.

He did not specify which two states will be looked into, but White House press secretary Sean Spicer, at a press briefing on Wednesday, mentioned the states of California and New York as examples of where voter fraud could have happened.

US TV network CNN quoted sources as saying that the investigation may start as early as Thursday.

Trump's remarks are seen as a follow-up on his previous claim that as many as 3 to 5 million people cast their votes illegally and he should have won the popular vote without those ballots from illegal immigrants.

Spicer said Trump's claim was based on research or data provided to him, but he did not give out details of the research.

Both Republicans and Democrats in the US Congress have countered Trump's statement. House Speaker Paul Ryan said Tuesday that he did not see any evidence of mass voter fraud, while Republican Senator Lindsey Graham advised Trump against making claims that are hard to be proved.

In the recent US presidential election, open records show Democratic nominee Hillary Clinton won the popular vote by almost 3 million votes, which made her the losing nominee with the biggest popular vote lead in US history.