Democrat Hillary Clinton's campaign will take part in a recount of Wisconsin votes in the U.S. presidential race, an effort Republican winner Donald Trump called "ridiculous".

Wisconsin's election board on Friday approved the recount requested by Green Party candidate Jill Stein. She has said she wants to guarantee the integrity of the U.S. voting system since computer hacking had marked the November 8 election.

Marc Elias, the Clinton campaign counsel, said the campaign would take part in the recount in Wisconsin as well as in the other battleground states of Pennsylvania and Michigan if recounts were mounted there.

Elias said in a statement on the Medium website that the Clinton campaign had not planned to seek a recount since its own investigation had failed to turn up any sign of hacking of voting systems.

"But now that a recount has been initiated in Wisconsin, we intend to participate in order to ensure the process proceeds in a manner that is fair to all sides," Elias said.

Clinton's campaign should be legally represented in Wisconsin to be able to monitor the recount, he said.

In a statement, Trump said the three states had been won by wide margins, including by more than 70,000 votes in Pennsylvania. The recount is a "scam by the Green Party for an election that has already been conceded," he said.

Although Trump won the Electoral College tally, Clinton will have won the national popular vote by more than 2 million ballots when the final results are in

The $7 million Stein has sought to raise for the recounts is a way "to fill her coffers with money, most of which she will never even spend on this ridiculous recount," he said.

Although Trump won the Electoral College tally, Clinton will have won the national popular vote by more than 2 million ballots when the final results are in.

Stein has raised $5.8 million of the $7 million needed to cover fees and legal costs for the three recounts, according to her campaign website. The deadline for filing a recount bid in Pennsylvania is Monday.

The voting margins make it highly unlikely any recounts would end up giving Clinton a win in all three states, which would be needed for the overall election result to change.

Trump beat Clinton in Pennsylvania by 70,010 votes, in Michigan by 10,704 votes and in Wisconsin by 27,257 votes.

Experts urged extra scrutiny of the three states, Stein told CNN on Friday, because their voting systems were seen as vulnerable.