(CNN) Several Republican lawmakers were more "shaken" than they publicly let on by the testimony of a US official in Kiev who overheard a phone conversation between the US ambassador to the European Union and President Donald Trump, a top GOP congressional source told CNN.

David Holmes -- who will testify publicly as part of the House impeachment inquiry -- said in closed-door testimony last week that US Ambassador to the EU Gordon Sondland had told Trump that Ukrainian President Volodymyr Zelensky "loves your ass" and that Ukraine was going to move forward with the investigation Trump had asked Zelensky for a day earlier. Holmes said he was able to overhear the conversation due to the volume of Trump's voice while he sat with Sondland at a restaurant in Kiev.

According to the GOP congressional source, that testimony led several GOP lawmakers to express frustration that Sondland would place a call to the President in a public restaurant, and are concerned that Holmes' testimony was the most convincing argument for Trump's direct involvement in the campaign to pressure Ukraine.

The House Intelligence Committee will have Holmes , the counselor for political affairs at the US Embassy in Ukraine, testifying alongside former White House official Fiona Hill on Thursday, according to the Democratic aide. The addition of Holmes means nine individuals will testify publicly as witnesses in the House impeachment probe this week.

Holmes' testimony is making some GOP members worry about how far Sondland will go in his public testimony on Wednesday and two senior Republican sources say that some House Republicans are worried about how Sondland will handle himself at the hearing.

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