Iran Press/America: In previously unheard testimony, Bill Taylor, the acting US ambassador to Ukraine, said a member of his staff was told the US President Donald Trump was preoccupied with pushing for a probe into Joe Biden.
He was speaking at the first public hearings in the impeachment inquiry, BBC reported.
Trump told reporters he did not recall making such comments.
Trump is accused of withholding US military aid to Ukraine in order to pressure the country's new president to publicly announce a corruption inquiry into Biden, among the favourites to take him on in the 2020 presidential race.
Trump denies any wrongdoing and has called the inquiry a "witchhunt".
During a detailed opening statement, Taylor said a member of his staff had overheard a telephone call in which the president inquired about "the investigations" into Biden.
The call was with Gordon Sondland, the US ambassador to the European Union, who reportedly told the president over the phone from a restaurant in Kyiv that "the Ukrainians were ready to move forward".
After the call, the staff member "asked ambassador Sondland what President Trump thought about Ukraine", Taylor said.
Taylor said: "Ambassador Sondland responded that President Trump cares more about the investigations of Biden."
Meanwhile observers and former officials have drawn attention to the security implications of making the call from a restaurant, potentially exposing the conversation to eavesdropping by Russian intelligence.
When asked about Sondland earlier this month, the president had said: "I hardly know the gentleman."
Media caption President Trump: "The witch-hunt? I hear it's a joke... I haven't watched for one minute"
Responding to queries from reporters after the hearing, Trump said: "I know nothing about that, first time I've heard it."
He said he recalled Sondland's testimony, in which the diplomat said he spoke to the president "for a brief moment" and Trump had "said no quid pro quo under any circumstances".
He did not recall the phone call Taylor described, "not even a little bit", and "in any event it's more second hand information", he said.
The impeachment inquiry has been going on for more than a month - but all previous hearings were private, with reports based on leaks and sources speaking to the media.
Wednesday's public hearings were the first time the public heard from witnesses directly and a chance for Democrats and Republicans to win over voters.
This has the potential to be a major twist. Although there have been reports of Sondland's direct line to the president, there has yet to be evidence tying Trump directly to the alleged quid pro quo.
The phone call Taylor described could change all that.
In the middle of Wednesday's hearing, the House Intelligence Committee announced a new witness scheduled to give a closed-door deposition on Friday, an aide named David Holmes - reportedly the aide Taylor mentioned.
Next week, Sondland himself is scheduled to testify during public hearings.
If either of these two men support Taylor's account, it could undercut the president's defenders who have suggested that Trump was not closely involved in the activities of the "unofficial" channel of Ukraine policy, as Mr Taylor called it, which was pressuring Ukraine to open up investigations into the Bidens.
Democrats have reason to be pleased, while the president's team has a new set of headaches.
House Democrats last month launched an impeachment inquiry after it was revealed that Trump had asked the Ukrainian President to investigate US Democratic candidate Joe Biden and his son.
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