McGahn, who served as Trump's presidential lawyer for nearly two years before resigning in October 2018, testified on Friday in a closed-door session before the US House of Representatives judiciary committee.

McGahn appeared under a subpoena issued about two years ago to testify as the committee was looking into allegations of wrong-doing by Trump.

Late in 2019, the House voted to impeach Trump on charges of abuse of power and obstruction of Congress.

He was acquitted by the Senate, then under Republican control.

A transcript of McGahn's testimony, which last almost eight hours, is due to be made public in coming days.

"Mr McGahn was clearly distressed by President Trump's refusal to follow his legal advice, again and again, and he shed new light on several troubling events today," committee chairman Jerrold Nadler said in a statement.

Republican Representative Matt Gaetz told reporters that McGahn's testimony provided no new information, however.

"The expectation was that Don McGahn would be some sort of essential witness bringing new information worthy of years of litigation and countless taxpayer dollars spent," Gaetz said of Democrats.

Representative Madeleine Dean, a senior judiciary committee Democrat, told reporters McGahn "brought to life the pressure he was under, the pressure that other aides were under by the president to direct Rod Rosenstein to oust special counsel (Robert) Mueller."

At the time, Rosenstein was serving as deputy attorney-general and Mueller was probing Trump and his 2016 presidential campaign.

After a lengthy investigation, Mueller found "numerous links" between the campaign and the Russians and concluded the campaign "expected it would benefit" from Moscow's effort to tilt the vote in Trump's favour.

But Mueller said such interactions either did not amount to criminal behaviour or would be difficult to prove in court.

Australian Associated Press