A US diplomat told Donald Trump Ukraine would carry out investigations the president had asked for, a US official testified at the impeachment inquiry.
State department official David Holmes said he had overheard this during a call in July between Mr Trump and the US envoy to the EU, Gordon Sondland.
Mr Holmes said the call came a day after Mr Trump asked Kyiv to probe ex-Vice-President Joe Biden.
Mr Trump has dismissed the impeachment inquiry as "presidential harassment".
The inquiry is investigating whether Mr Trump withheld US military aid to Ukraine in order to pressure the country's new President Volodymyr Zelensky to announce a corruption inquiry into Mr Biden, now his rival for the US presidency.
On Friday, Mr Trump launched a Twitter attack on another witness - former US ambassador to Ukraine Marie Yovanovitch.
"Everywhere Marie Yovanovitch went turned bad," Mr Trump wrote in the middle of her testimony. "She started off in Somalia, how did that go?"
Asked for her response, Ms Yovanovitch called it "very intimidating".
Mr Trump later hit back, arguing his tweets were not intimidating "at all". He told reporters he had watched part of the impeachment hearing and considered it "a disgrace".
WARNING: This report contains strong language.
What did David Holmes say?
Mr Holmes, a US diplomatic aide, testified behind closed doors before lawmakers in Washington DC.
He said he had overheard the phone call between Mr Trump and Mr Sondland in which "investigations" are said to have been discussed.
He said Mr Sondland called Mr Trump from a restaurant in Ukraine's capital Kyiv on 26 July 2019.
According to a copy of his opening statement obtained by CBS News, Mr Holmes said: "Sondland told Trump that [Ukrainian President] Zelensky 'loves your ass.'"
"I then heard President Trump ask, 'So, he's gonna do the investigation?'
"Ambassador Sondland replied that 'he's gonna do it', adding that President Zelensky will do 'anything you ask him to'."
Observers have drawn attention to the security implications of making the call from a restaurant, potentially exposing the conversation to eavesdropping by Russian intelligence.
Mr Holmes' deposition appears to corroborate Wednesday's testimony to the impeachment inquiry by US ambassador to Ukraine Bill Taylor.
He said one of his aides heard the same chat.
The aide said Mr Trump had asked about "investigations" and Mr Sondland had replied that Ukraine was ready to move forward.
According to Mr Taylor, Mr Sondland then told the aide that the president cared more about the investigation of the Bidens than anything else involving Ukraine.
The call - which the US president has denied any knowledge of - allegedly happened the day after the now-famous Trump-Zelensky phone call.
What about the Trump-Yovanovitch row?
While giving her evidence, Ms Yovanovitch was alerted to the president's criticism by the hearing's chairman Adam Schiff.
Responding directly to Mr Trump's tweet, in which he appeared to blame her for upheaval in Somalia, Ms Yovanovitch replied: "I don't think I have such powers, not in Mogadishu and Somalia and not in other places.
"I actually think that where I've served over the years I and others have demonstrably made things better, you know, for the US as well as for the countries that I've served in.
Her response was broadcast live during the televised hearing.
Mr Schiff, the Democratic Chairman of the Intelligence Committee overseeing the impeachment inquiry, suggested the president's tweets could be classed as witness intimidation.
Ms Yovanovitch was removed as ambassador to Kyiv in May, two months before a controversial phone call between Mr Trump and his Ukrainian counterpart Volodymyr Zelensky, which is now key to the inquiry.
A rough transcript of the call revealed that Mr Trump had urged President Zelensky to investigate unsubstantiated allegations against Mr Biden and his son Hunter, who sat on the board of a Ukrainian gas company.
In earlier closed-door testimony, Ms Yovanovitch alleged she had fallen victim to a smear campaign at the hands of Mr Trump's personal lawyer Rudy Giuliani.
She said Mr Giuliani had worked to discredit her while attempting to push Ukraine into the anti-Biden investigations.