One top national security aide who listened to President Donald Trump’s July call with Ukraine’s president called it “improper” while another said it was “unusual.”

The two gave evidence on Tuesday at House impeachment hearings as the inquiry reached deeper into 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 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”.

He did so, he said, “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”.

In the audience was his twin brother, also an official at the National Security Council and among those he told about his concerns over Mr Trump’s phone call.

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

In all, nine current and former US officials are giving evidence as the House’s impeachment inquiry accelerates.

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.

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Jennifer Williams (Andrew Harnik/AP)Gavelling open the 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.”

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National Security Council aide Lieutenant Colonel Alexander Vindman (Andrew Harnik/AP)

On both occasions, Lt Col Vindman said, he took his concerns about the shifting Ukraine policy to the lead counsel at the NSC, John Eisenberg.

Ms Williams, a career State Department official who has worked for three presidential administrations and counts former US secretary of state Condoleezza Rice as a “personal hero”, said she too had concerns during the phone call, which the aides monitored as is standard practice.

When the White House produced a rough transcript later that day, she put it in the vice president’s briefing materials.

“I just don’t know if he read it,” Ms Williams testified earlier in her closed-door House interview.

Mr Pence’s role throughout the impeachment inquiry has been unclear, and the vice president’s aide was questioned by politicians looking for answers.

Mr Trump has assailed Ms Williams, associating her with “Never Trumpers”, even though there is no indication she has shown any partisanship.

In the second hearing of the day, both witnesses were requested by Republicans.

Tim Morrison, who stepped down from the National Security Council shortly before he appeared behind closed doors last month, said his recent resignation from the Trump administration was voluntary.

He added that he felt no pressure to resign and did not receive any retaliation for his closed-door testimony to Congress last month.

Former special envoy to Ukraine Kurt Volker, meanwhile, was the first person to come behind closed doors in the inquiry that started in September, resigning his position shortly before he did so.

He testified that Mr Biden “respects his duties of higher office” and it is not credible that he would act in any way other than in the national interest.