Donald Trump's phonecall with Ukraine's president was described as "improper" by a national security aide listening in on the call, while another said it was "unusual" at the US president's impeachment hearing.

The two advisers gave evidence at the House hearing, as impeachment proceedings against Trump continue to probe into his dealings at the White House.

Lieutenant Colonel Alexander Vindman, an Army officer at the National Security Council, and Jennifer Williams, his counterpart at Vice President Mike Pence’s office, said they had concerns as Mr Trump spoke on July 25 with the newly elected Ukraine president Volodymyr Zelenskiy about political investigations into Democrat Joe Biden.

“What I heard was inappropriate,” Lt Col Vindman told politicians.

Ms Williams said: “I found the July 25th phone call unusual because, in contrast to other presidential calls I had observed, it involved discussion of what appeared to be a domestic political matter.”

Lt Col Vindman, a 20-year military officer arrived at Capitol Hill in military blue with a chest full of service medals, and said he reported his concerns “out of a sense of duty”.

Vindman said he reported the calls "because they had significant national security implications for our country".

An immigrant, who arrived in the US as a toddler from Ukraine, Lt Col Vindman told the panel he was grateful his father brought the family to the US 40 years ago and for “the privilege of being an American citizen and public servant, where I can live free of fear for mine and my family’s safety”.

Vindman's twin brother, also an official on the National Security Council, was among those he expressed concerns to about Trump's phone call.

Jennifer Williams gave evidence at Trump's impeachment hearing on Tuesday. Credit: Andrew Harnik/AP

Addressing his late father, he said: “Do not worry, I will be fine for telling the truth.”

What is the impeachment process about?

Mr Trump is facing impeachment over allegations he acted improperly in calls with the Ukrainian president.

Nine current and former US officials are giving evidence.

Democrats say Mr Trump’s pressure on Ukraine to investigate his Democratic rivals as he withheld US military aid Ukraine needed to resist Russian aggression may be grounds for removing the 45th president.

Mr Trump says he did no such thing and the Democrats just want him gone.

National Security Council aide Lieutenant Colonel Alexander Vindman. Credit: Andrew Harnik/AP

At the opening of second week of live televised hearings, the Democratic chairman leading the probe, Representative Adam Schiff, noted that Mr Trump tweeted against Ms Williams over the weekend and Lt Col Vindman has seen “far more scurrilous attacks” on his character by the president’s allies.

Mr Schiff, who has warned that the president’s attacks on others in the impeachment inquiry could be seen as intimidation, said the witnesses “are here because they were subpoenaed to appear, not because they are for or against impeachment. That question is for Congress”.

The top Republican on the House Intelligence Committee, Representative Devan Nunes, began the hearing with an extended attack on the media and dismissed last week’s evidence as “second-hand and third-hand conversations”.

He blasted the hearing as a “hoax”.

Lt Col Vindman and the other witnesses have given evidence in earlier, closed-door sessions.

Their depositions have been publicly released.

“I did not think it was proper to demand that a foreign government investigate a US citizen,” said Lt Col Vindman.

He said there was “no doubt” what Mr Trump wanted from Ukrainian President Volodymyr Zelenskiy.

It was not the first time Lt Col Vindman, a decorated Iraq War veteran, was alarmed over the administration’s push to have Ukraine investigate Democrats, he said.

Earlier, during an unsettling July 10 meeting at the White House, Ambassador Gordon Sondland told visiting Ukraine officials that they would need to “deliver” before next steps, which was a meeting Mr Zelenskiy wanted with Mr Trump, the officer testified.

“He was talking about the 2016 elections and an investigation into the Bidens and Burisma,” Lt Col Vindman said, referring to the gas company in Ukraine where Biden’s son Hunter served on the board.

“The Ukrainians would have to deliver an investigation into the Bidens,” he said.

“There was no ambiguity.”