U.S. President-elect Donald Trump said without evidence in a tweet on Sunday that he won the popular vote in the Nov. 8 election "if you deduct the millions of people who voted illegally."

The allegation by Trump, who won the required votes in the Electoral College to secure the presidency, comes as Democratic rival Hillary Clinton's lead in the popular vote over Trump has surpassed two million votes and is expected to grow to more than 2.5 million as ballots in populous states such as California continue to be tallied.

Clinton's legal team said on Saturday it had agreed to participate in a recount of Wisconsin votes after the state's election board approved the effort requested by Green Party candidate Jill Stein, which Trump has called "ridiculous."

"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," the Republican Trump tweeted as reporters waited for him to leave his Mar-a-Lago golf resort in Florida to fly back to his residence in New York City.

In a subsequent tweet, he alleged that "serious" voter fraud had occurred in California, New Hampshire and Virginia, and that the media were ignoring the story.

Those three states were won by Clinton.

The source of the claims is unclear, although the conspiracy-minded website Infowars posited a week after the election, without supporting evidence, that widespread voter fraud had occurred.

Conflicting messages

The U.S. presidential race is decided by the Electoral College, based on a tally of wins from the state-by-state contests, rather than by the national vote. Trump has surpassed the 270 electoral votes needed to win the White House. The Electoral College results are expected to be finalized on Dec. 19. Trump takes office on Jan. 20.

"It would have been much easier for me to win the so-called popular vote than in the Electoral College in that I would only campaign in three of four states instead of the 15 states that I visited," Trump added in follow-up tweets.

Voters line up on election day in Woodstock, N.H. Trump alleged Sunday, without supporting evidence, that New Hampshire was one of three states where widespread voter fraud occurred. (Jim Cole/The Associated Press)

Before the election, Trump made unsubstantiated allegations that the results of the election might be "rigged" against him.

Since the vote, his message has alternated between appealing for unity and railing against his opponents and the media.

In a video message released ahead of the U.S. Thanksgiving holiday on Thursday, Trump said he hoped it would be a time for Americans "to begin to heal our divisions" following a "long and bruising political campaign."

Trump has derided the fundraising effort by Stein to launch recounts in Wisconsin, Michigan and Pennsylvania as a "scam." Those states had voted Democratic in recent presidential elections but all broke narrowly for the Republican Trump in this month's election. The recounts are not expected to change the results of the election.

Stein, who won about one per cent of the national vote, has said she wants a recount to guarantee the integrity of the U.S. voting system, a push that came after some experts raised the possibility that hacks could have affected the results.

Democratic President Barack Obama's administration has said there is no evidence of electoral tampering, but experts have said that the only way to verify the results are accurate is to conduct a recount.