WASHINGTON (AFP) - US lawmakers return to Washington Friday (Jan 3) with US President Donald Trump's upcoming impeachment trial looming over the first Senate session of 2020, but congressional leaders are locked in stalemate over how to proceed.

As Congress braces for a frenetic January, newly uncovered evidence could bolster Democrats' demands for testimony from key witnesses with firsthand knowledge of Trump's Ukraine pressure scheme at the heart of the impeachment scandal.

A series of emails published Thursday signal that the order to maintain a freeze on military aid to Ukraine came directly from Trump, and reveal that the Pentagon raised serious concerns about the legality of the directive.

House Speaker Nancy Pelosi has so far refused to transmit the two articles of impeachment against Trump - abuse of power and obstruction of Congress - to the Senate until the chamber agrees on trial parameters that she considers fair.

No progress was made over the holiday break as to the rules of the trial, after Trump last month became only the third US president ever impeached by the House of Representatives.

Senate Democrats want to hear from key witnesses who refused to testify during the House investigation, and obtain documents denied to the impeachment probe.

Republicans have balked, with Senate Majority Leader Mitch McConnell admitting last month that he was working in "total coordination" with the White House on how to conduct the trial.

Trump stands accused of leveraging a highly sought White House meeting and US$391 million in military aid to Ukraine, which Kiev desperately needed to deter Russian aggression, in exchange for investigations into Democrats including his possible 2020 election rival Joe Biden.

The White House has rejected that narrative.

But on the eve of the Senate re-convening, unredacted emails published by a national security site show that a White House official told the Pentagon that the order to hold the Ukraine aid came from the president himself.

According to Just Security, an August 30 email sent by Michael Duffey, associate director of national security programmes at the Office of Management and Budget, to the Pentagon's comptroller said the aid freeze would continue at Trump's direction, despite mounting legal worry within the Defense Department.

"Clear direction from POTUS to continue to hold," Duffey wrote in the email.

The unredacted message was one of several not turned over to House investigators conducting the impeachment inquiry, Just Security reported.

On Thursday, top Senate Democrat Chuck Schumer said Duffey's email "further implicates" Trump and compromises McConnell's push to have a trial without documents and witnesses as sought by Democrats.

Schumer said it was imperative that Duffey and other key figures testify, including Secretary of State Mike Pompeo and acting White House chief of staff Mick Mulvaney.

Pelosi also weighed in. "Trump engaged in unprecedented, total obstruction of Congress, hiding these emails, all other documents, and his top aides from the American people," she tweeted.

"Why won't Trump & McConnell allow a fair trial?" Trump on Thursday repeated his claim that the impeachment effort was a "partisan witch hunt" that has fuelled national divisions.