David Holmes, a political adviser at the US Embassy in Ukraine, will testify on Thursday as part of the House of Representatives' impeachment inquiry of President Donald Trump, media reported.
Holmes has been scheduled to appear before the House Intelligence Committee on Thursday for a public impeachment hearing, Axios reported on Monday.
On Friday, Holmes told House lawmakers that he overheard a telephone call between Trump and US Ambassador to the European Union Gordon Sondland the day after the US president's July 25 call with Ukrainian President Volodymyr Zelenskyy.
Trump asked Gordon if Ukraine was going to open an investigation into former Vice President Joe Biden - a political rival in the 2020 election - whose son was once a board member at Ukrainian gas company Burisma; the ambassador confirmed they would.
The freeze of hundreds of millions of dollars in US military aid to Ukraine this summer has been at the core of the impeachment inquiry against Trump's alleged abuse of power. The Democrats believe that Trump held back aid to Ukraine to pressure its leadership into opening an investigation into.
Trump has categorically denied applying any pressure to the Ukrainian leadership when requesting it to probe possible instances of corruption, which he says is his duty as president.
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