A top Democrat formally requested on Wednesday that the US tax agency provide Congress with President Donald Trump's tax returns, setting up a likely legal battle with the White House.

Richard Neal, chairman of the House Ways and Means Committee which writes tax law, wrote the Internal Revenue Service seeking six years of Trump's returns, saying he was using his oversight authority to learn the extent to which the IRS audits and enforces tax laws on a sitting president.

"Under the Internal Revenue Manual, individual income tax returns of a president are subject to mandatory examination, but this practice is IRS policy and not codified in the Federal tax laws," Neal wrote.

He told IRS Commissioner Charles Rettig to deliver the president's individual income tax returns, as well as those from eight entities connected to Trump, by April 10.

The president signalled he would not want his returns provided to Congress.

"We are under audit despite what people say," Trump told reporters, repeating his 2016 campaign excuse for refusing to release his returns.

"Until such time as I am not under audit I would not be inclined" to release them, he added.

Treasury Secretary Steven Mnuchin said last month he would "follow the law" if Congress requested Trump's tax returns.

Trump, a self-declared billionaire, released a less detailed financial disclosure in 2017 but has kept his tax returns private, fuelling speculation about his revenue sources and net worth.

Trump is the first president since Richard Nixon to refuse to release his returns.

Congressman Dan Kildee, a Ways and Means Committee Democrat, said there was "no question" the panel has legal authority to gain access to Trump's records.

"We expect the IRS commissioner to comply with the law and provide those returns," Kildee told CNN.

In a statement, the committee said that under IRS guidelines, a president's tax returns are subject to "mandatory examination."

Kildee stressed that the request had nothing to do with the timing of the investigation into Russia's interference in the 2016 US election, and the report recently submitted by special counsel Robert Mueller.

"This has nothing to do with the Mueller report," Kildee said.

"Obviously there are very serious other public interests that would be served by having access, but principally this is about the policy question."

The panel's top Republican Kevin Brady rushed to Trump's defence, writing Mnuchin to warn of Democrats' "misguided rush to impeach" Trump.

"Weaponizing our nation's tax code by targeting political foes sets a dangerous precedent and weakens Americans privacy right," he wrote.

The committee said in its statement that Neal insisted the request "is not about impeachment," but legitimate oversight issues.