Democratic lawmakers gave tax authorities a final deadline of 23 April to hand over US President Donald Trump's tax returns, which Treasury Secretary Steven Mnuchin said the administration was considering.

Democrats have seized on the issue of Mr Trump's undisclosed taxes since taking control of the House of Representatives - citing a little-known law that allows Congress to review anyone's returns to conduct an investigation.

"I am aware that concerns have been raised regarding my request and the authority of the Committee," House Ways and Means Committee Chairman Richard Neal said in a letter to Internal Revenue Service Commissioner Charles Rettig after an initial 10 April deadline lapsed. "Those concerns lack merit."

On Wednesday, Mr Mnuchin said the Treasury was handling the request and that a deadline set for that date would be missed, citing possible constitutional issues.

Yesterday however, he said the administration would respond by the revised deadline.

"I'm sure we'll respond by that deadline, not going to make a commitment prematurely whether we'll be able to conclude a legal review by that deadline," Mr Mnuchin said. "We have people working on it diligently."

Calling Mr Neal's new deadline an "arbitrary" date, Mr Mnuchin said the Democrats' request could set "enormous precedent in potentially weaponizing the IRS".

Donald Trump during the 2016 presidential campaign

President Trump argues that he cannot release his tax returns because they are being audited, but the IRS has said this is no impediment to their release.

"It is not the proper function of the IRS, Treasury or Justice to question or second guess the motivations of the Committee or its reasonable determinations regarding its need for the requested tax returns and return information," Mr Neal said.

"Concerns about what the Committee may do with the tax returns and return information are baseless."

Mr Neal’s deadline for the US government to hand over the tax records is on 23 April.

"Please know that, if you fail to comply, your failure will be interpreted as a denial of my request," Mr Neal wrote.

The request looks set to trigger a protracted legal showdown.

President Trump broke with a long-established norm during the 2016 elections by refusing to release his income tax returns, as most presidents have done since the 1970s even though it is not required by law.

His acting chief of staff, Mick Mulvaney, asked last week whether Democrats would ever see the returns, was unequivocal.

"Never," he said on Fox News Sunday. "Nor should they."

Mr Mulvaney asserted that since voters knew ahead of the 2016 election that Donald Trump would not reveal his returns and then elected him anyway, the matter was "already litigated."

Read more:
Reports suggest Donald Trump avoided paying tax for up to 18 years