US President Donald Trump's top adviser on Ukraine is set to testify on Tuesday that he told a government lawyer about concerns that US national security could be undermined after a phone call between Trump and Ukraine's president.Alexander Vindman, an army lieutenant colonel and former diplomat who later became the director of European affairs on the National Security Council (NSC), said he had listened in on Trump's July 25 call with Ukrainian President Volodymyr Zelenskyy from the Situation Room with fellow NSC officials. Vindman is the first official to testify who directly listened in on the call, in which Trump urged Zelenskyy to investigate the Burisma gas-company, which is linked to former Vice President Joe Biden's son, Hunter. Joe Biden is a front-runner for the 2020 Democratic presidential nomination. Trump made the request after withholding nearly $400m in military aid approved by the US Congress to help fight Russian-backed separatists in eastern Ukraine.Vindman, who is also the first current White House official to appear before the impeachment panels, said he reported his concerns about the call, and its effects on national security, to the NSC's lead lawyer."I was concerned by the call," Vindman said in his prepared testimony. "I did not think it was proper to demand that a foreign government investigate a US citizen, and I was worried about the implications for the US government's support of Ukraine.""I realised that if Ukraine pursued an investigation into the Bidens and Burisma, it would likely be interpreted as a partisan play which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained. This would all undermine US national security," he continued.House Democrats launched the impeachment inquiry in September after a whistle-blower complaint about the July 25 call. They are investigating whether Trump withheld the military aid to pressure Ukraine to investigate Burisma and if he sought the help of a foreign government for a US election, which is illegal under federal law.Vindman, a 20-year military officer and decorated veteran, said he is not the whistle-blower who made the initial complaint.There is, to date, no evidence of wrongdoing by the Bidens.Denying any wrongdoing, Trump has said his call with Zelenskyy was "perfect". On Twitter on Tuesday, Trump called the House inquiry a "sham" and characterised Vindman as a "never Trumper", or an official who has vowed never to support Trump. "Why are people that I never even heard of testifying about the call. Just read the call transcript and the impeachment hoax is over," he wrote.Previous report of concernThe phone call between Trump and Zelenskyy was not the first time Vindman felt compelled to report his concerns about the administration's dealings with Ukraine to the NSC attorney, according to his testimony.He also made a similar complaint following a July 10 meeting with US Ambassador to the European Union Gordon Sondland, he said.During the meeting, Sondland, who testified before a House panel last week, stressed the importance of having Ukraine investigate Burisma as well as now debunked claims that Ukraine may have meddled in the 2016 election, according to Vindman's testimony.Vindman said he told Sondland that "his statements were inappropriate, that the request to investigate Biden and his son had nothing to do with national security, and that such investigations were not something the NSC was going to get involved in or push".The account stands in stark contrast with the testimony delivered by Sondland to House investigators last week.Sondland, a hotelier who donated $1 million to Trump's candidacy and was appointed to the prestigious post by the president in 2018, said he did not recall that anyone from the NSC had "ever expressed any concerns" over the administration's dealings with Ukraine and that Sondland did not at the time know there was any connection between Biden and Burisma company.In his testimony, Vindman, who was wounded in Iraq and awarded a Purple Heart, described himself as a "patriot" who is fulfilling a "sacred duty and honour" to defend the US "irrespective of party or politics"."For over 20 years as an active duty United States military officer and diplomat, I have served this country in a nonpartisan manner, and have done so with the utmost respect and professionalism for both Republican and Democratic administrations," he wrote.Vote set on impeachmentVindman's testimony, which is expected to be key in the investigation, comes a day after House Speaker Nancy Pelosi announced the House would vote on a resolution to affirm the impeachment investigation, set rules for public hearings and outline the potential process for writing articles of impeachment against Trump. The vote is expected on Thursday.The move aims to nullify complaints from Trump and his allies that the impeachment inquiry is illegitimate, unfair and lacking due process. The White House had cited a lack of a vote on the inquiry, among other reasons, as justification for the administration not complying with the probe. Pelosi's announcement Monday came just hours after a former White House national security official defied a House subpoena for closed-door testimony, escalating the standoff between Congress and the White House over who will testify.Charles Kupperman, who was a deputy to National Security Advisor John Bolton, failed to show up for the scheduled closed-door deposition after filing a lawsuit asking a federal court in Washington, DC, to rule on whether he was legally required to appear.