The White House attacked its own top Ukraine official as he testified to an impeachment hearing that a phone call made by the president was "improper" and had left him in "shock", Eurasia Diary reports citing BBC.

Lt Col Alexander Vindman told Congress that President Donald Trump made "inappropriate" political demands of the Ukrainian president.

"I couldn't believe what I was hearing," Lt Col Vindman said.

The hearings are investigating whether Trump abused his presidential power.

A decorated Iraq war veteran who serves in a senior role on the US National Security Council (NSC), Lt Col Vindman testified before the House on Tuesday in his Army dress uniform.

As he described his reaction to hearing a call between President Trump and his Ukranian counterpart, President Volodymyr Zelensky, Lt Col Vindman came under attack by the official White House Twitter account, which posted a critical quote from his former boss on the NSC questioning his judgement.

Republican congressmen pressed Lt Col Vindman on the remark and questioned his loyalty to the US - asking about instances in which Ukrainian officials approached him about becoming the country's defence minister.

"Every single time I dismissed it," Lt Col Vindman said. "I'm an American."

Lt Col Vindman told the US House intelligence committee that he had been concerned by the president's demands to investigate Biden.

"It was improper for the president to request - to demand - an investigation into a political opponent, especially [from] a foreign power where there is at best dubious belief that this would be a completely impartial investigation and that this would have significant implications if it became public knowledge," he told the committee.

He had reported the "inappropriate" discussion to NSC lawyers "out of a sense of duty".

The official Twitter account of the White House was used throughout the hearing to attack Lt Col Vindman and the inquiry in real time, retweeting hashtags including #ShamImpeachment and #ParodyImpeachment.