The top US diplomat in Ukraine, testifying in the first televised hearing of the impeachment inquiry against Donald Trump, has linked the US President more directly to a pressure campaign on Ukraine to conduct investigations that would benefit him politically.

William Taylor was one of two career diplomats who testified before the US House of Representatives Intelligence Committee on Wednesday as a crucial new phase began in the impeachment inquiry that threatens Mr Trump’s presidency even as he seeks re-election in 2020.

Both Mr Taylor and George Kent testified about their concerns about pressure by Mr Trump and allies to get Ukraine to investigate Democratic political rival Joe Biden in a dramatic hearing that pitted Democratic and Republican politicians against each other.

An important disclosure came from Mr Taylor, the acting ambassador to Ukraine. He pointed to the Republican President’s keen interest in getting Ukraine to investigate Mr Biden, a former vice-president, and reiterated his understanding that $US391 million in US security aid was withheld from Kiev unless it cooperated.

Mr Taylor said a member of his staff overheard a July 26 phone call between Mr Trump and Gordon Sondland, a former political donor appointed as a senior diplomat, in which Mr Trump asked about those investigations and Mr Sondland told him the Ukrainians were ready to proceed.

Following the call – which came a day after Mr Trump had asked Ukraine’s president during a phone call to conduct the investigations – the employee asked Mr Sondland, the US ambassador to the European Union, what Mr Trump thought about Ukraine, Mr Taylor said.

“Ambassador Sondland responded that President Trump cares more about the investigations of Biden, which Giuliani was pressing for,” Mr Taylor testified, referring to Mr Trump’s personal lawyer, Rudy Giuliani.

Asked by Adam Schiff, the committee’s Democratic chairman, if that meant MR Trump cared more about the investigations than about Ukraine, Taylor said: “Yes, sir.”

Republican politicians called Mr Taylor’s account hearsay and noted Ukraine’s president has not said he felt pressured by Mr Trump.

With a potential television audience of tens of millions looking on, Mr Schiff opened the historic session – the first impeachment drama in two decades – in an ornate hearing room packed with journalists, politicians and the public.

NEW: Taylor discloses that his staff overheard phone call in which Trump asked Sondland about investigations into the Bidens in July

Taylor's staff member says Sondland told them that "Trump cares more about the investigations into the Bidens" than anything else re: Ukraine pic.twitter.com/yEaEOzIAMn

— Aaron Rupar (@atrupar) November 13, 2019

The public hearing was the first chance for America, and the rest of the world, to see and hear for themselves details of the impeachment probe.

Mr Schiff’s accusations that Mr Trump abused his power was met by a staunch denial by the panel’s senior Republican, Devin Nunes, that Mr Trump and his aides improperly pressured Ukraine to dig up dirt on Mr Biden, a leading contender for the Democratic nomination for the 2020 election.

The focus of the inquiry is on the July 25 telephone call in which Trump asked Ukrainian President Volodymyr Zelenskiy to open a corruption investigation into Mr Biden and his son Hunter and into a discredited theory that Ukraine, not Russia, meddled in the 2016 US election. Hunter Biden had been a board member for a Ukrainian energy company called Burisma.

Mr Taylor said at the outset: “I am not here to take one side or the other, or to advocate for any particular outcome of these proceedings.”

"They're trying to stop me, because I'm fighting for you. And I'll never let that happen." — President @realDonaldTrump pic.twitter.com/ch0N1SWShe

— The White House 45 Archived (@WhiteHouse45) November 13, 2019

Mr Taylor is now the charge d’affaires of the US Embassy in Kiev. Mr Kent oversees Ukraine policy at the State Department.

“I do not believe the United States should ask other countries to engage in selective, politically associated investigations or prosecutions against opponents of those in power, because such selective actions undermine the rule of law regardless of the country,” Mr Kent said.

Mr Trump told reporters he had not watched the hearing because he was “too busy” but took aim at the House Intelligence Committee lawyers.

“I see they’re using lawyers that are television lawyers – they took some guys off television. I’m not surprised to see it because Schiff can’t undo his own questions,” he said.

He described the inquiry as a “witch hunt” and a “hoax”.

-with AAP