House Democrats unveiled a resolution on Tuesday authorizing and laying out procedures for the next phase of the impeachment inquiry against Donald Trump, saying it will provide “a clear path forward” as the House begins a public phase of the investigation.

“This is a sad time for our country,” said Democratic congressman James McGovern, the chairman of the House Rules Committee. “None of us came to Congress to impeach a president, but each of us took a solemn oath to protect and defend the constitution.”

The resolution came on a day of alarming testimony regarding Trump’s 25 July call with the Ukranian president, Volodymyr Zelenskiy, which was central to the whistleblower complaint that sparked the impeachment inquiry.

On Tuesday, Alexander Vindman, a former army colonel and diplomat, told House investigators that he listened to the Trump-Zelenskiy call and escalated his concerns.

President Trump is accused of trying to pressure Ukraine into investigating unsubstantiated corruption claims against his political rival, Joe Biden, and his son who worked with Ukrainian gas company Burisma.

Mr Trump denies wrongdoing and calls the impeachment inquiry a "witch hunt".

On Tuesday, the impeachment inquiry heard from Lt Col Alexander Vindman, a White House official who had monitored a phone call on 25 July between Mr Trump and Ukrainian President Volodymyr Zelensky.

That call sparked a whistleblower complaint and led to the impeachment probe.

Col Vindman said he was "concerned" by the call as he "did not think it was proper to demand that a foreign government investigate a US citizen".

Some details of the call were omitted from the official transcript, despite his attempts to have them included, Col Vindman added, according to US media citing sources.