US President-elect Donald Trump said in a tweet on Sunday that he won the popular vote in the November 8 election "if you deduct the millions of people who voted illegally", though he provided no evidence of widespread voter fraud.

The allegation by Mr 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 him 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.

Mrs 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 Mr Trump has called "ridiculous."