A top adviser to President Donald Trump on Ukraine testified on Tuesday that after listening to Trump ask Ukraine’s President to probe a domestic political rival he was so alarmed that he reported the matter to a White House lawyer out of concern for US national security.

Army Lieutenant Colonel Alexander Vindman, director of European affairs on the National Security Council, arrived at the US Capitol clad in his military dress uniform as he became the first current White House official to testify in the House of Representatives impeachment inquiry against Trump.

Vindman also became the first person to testify who listened in on the July 25 call at the heart of the Ukraine scandal. Even before his arrival, some allies of the Republican President, including Fox News host Laura Ingraham, sought to attack Vindman’s integrity and questioned his loyalty to the US.

Biden defended Vindman as a hero, calling attacks on the army officer's character and loyalty “despicable”.

“He’s a hell of a patriot,” the former Vice-President told MSNBC.

“I was concerned by the call,” Vindman said in his opening statement to the three House committees conducting the Democratic-led impeachment inquiry. “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.”

During the call, Trump asked Ukrainian President Volodymyr Zelenskiy to investigate former Vice-President Joe Biden, a Democratic political rival, and his son Hunter Biden, who had served on the board of Ukrainian gas company Burisma. Trump also asked Zelenskiy to investigate a debunked conspiracy theory that Ukraine, not Russia, interfered in the 2016 US election.

Vindman, who appeared after receiving a subpoena from lawmakers, recounted listening in on the call. “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 said in his testimony.

After the call, Vindman added, he reported his concerns to the National Security Counsel’s lead counsel. The call prompted a complaint from an intelligence community whistleblower, whose identify has not been revealed, that triggered the impeachment inquiry. In his statement, Vindman denied being the whistleblower or knowing the identity of the individual.

At a July 10 meeting in Washington with visiting Ukrainian officials, Vindman said US ambassador to the European Union, Gordon Sondland, a former Trump political donor, told the Ukrainian officials they needed to “deliver specific investigations in order to secure a meeting with the President”. At that point, Vindman said, then national security adviser John Bolton cut the meeting short.

According to Vindman’s prepared remarks, Sondland told other US officials in a debriefing after the meeting that it was important that the Ukrainian investigations centre on the 2016 election, the Bidens and Burisma.

“I stated to Amb. Sondland that his statements were inappropriate, that the request to investigate Biden and his son had nothing to do with national security,” Vindman

said, adding he also reported his concerns to the National Security Counsel’s lead lawyer.

Trump denies any wrongdoing. “Why are people that I never even heard of testifying about the call. Just READ THE CALL TRANSCRIPT AND THE IMPEACHMENT HOAX IS OVER! Ukrain (sic) said NO PRESSURE,” he tweeted on Tuesday.