US President Donald Trump’s campaign says it will seek a recount of votes in Wisconsin, hours before CNN and the Associated Press projected Democrat Joe Biden had won the key battleground state’s 10 electoral college votes.

“There have been reports of irregularities in several Wisconsin counties which raise serious doubts about the validity of the results,” Trump campaign manager Bill Stepien said in a statement, without providing details of any reports.

“The President is well within the threshold to request a recount and we will immediately do so.”

Election experts say fraud is very rare in US voting.

Edison Research said the margin between Trump and Biden in Wisconsin was less than one percentage point, allowing a candidate to seek a recount.

It said Biden had a slight lead in the state, with 99 per cent of the expected votes tallied so far. The former vice president had 49.4 per cent of the vote, while Trump had 48.8 per cent.

The CNN and AP projections would raise Biden’s tally of electoral votes, but several states have not been called for either candidate.

The CNN tally puts Biden at 234 of the 270 votes needed to win the White House, versus 213 for Trump. AP’s tally stands at 238 to 213.

Meagan Wolfe, Wisconsin’s elections chief, said on Wednesday the state’s electoral officials were re-checking results after working through the night to count valid ballots.

“Wisconsin’s counting and reporting of unofficial results has gone according to law,” she said in a statement.

“Today, the Wisconsin Elections Commission staff will be standing ready to assist clerks as they start the process of triple-checking the results.”

That process would include randomly selecting five per cent of reporting units for voting equipment audits required before results can be certified on December 1, she said.

No comment was immediately available from Wisconsin officials about whether the Trump campaign had submitted a formal recount request.