A top US official has said a phone call the US president made to ask Ukraine to investigate political rivals was "improper".
Lt Col Alexander Vindman was testifying before the US House intelligence committee as part of the impeachment inquiry into Donald Trump.
He said Mr Trump had made inappropriate political demands of his Ukrainian counterpart during a July phone call.
The inquiry is investigating whether presidential power was abused.
It seeks to establish whether Mr Trump withheld US military aid to Ukraine in order to pressure the country's new leader, Volodymyr Zelensky, into announcing a corruption inquiry into Joe Biden, Mr Trump's Democratic rival for the US presidency.
What did Vindman say?
The White House National Security Council (NSC)'s top Ukraine expert testified that he had been concerned by the president's demands to investigate Mr Biden.
"It was inappropriate, 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".
Lt Col Vindman was amongst the US officials who listened in on the 25 July call between the two leaders.
He is a decorated Iraq war veteran and was born in Ukraine; his father made the decision 40 years ago to move his family out of the Soviet Union to go and live in the US.
Who else testified on Tuesday?
The other witness in the morning session was Jennifer Williams, a foreign service adviser to Vice-President Mike Pence.
She said Mr Trump's reference to Mr Biden in the 25 July call with Mr Zelensky had been "unusual" because it delved into domestic US politics.
"The reference to Biden sounded political to me," she said.
The inquiry's afternoon session heard from Tim Morrison, the former top Russia and Europe expert on the National Security Council and Kurt Volker, former US special representative to Ukraine.
Mr Volker told the hearing President Trump "had a deeply rooted negative view on Ukraine rooted in the past" and despite "positive news and recommendations" being conveyed about Ukraine's new president "he was clearly receiving other information from other sources, including Mayor Giuliani, that was more negatiRead More – Source