WASHINGTON (AFP) -- Two top US diplomats delivered gripping testimony Wednesday about Donald Trump's efforts to get Ukraine to investigate his potential 2020 rival Joe Biden, as the impeachment inquiry into the president shifted into a new phase of high-stakes televised hearings.

Trump dismissed the probe in the Democratic-led House of Representatives as a "witch hunt" and said he was "too busy" to watch the first public hearings, during which he received staunch backing from lawmakers of his Republican Party.

William Taylor, the top US diplomat in Ukraine, began his testimony before the House Intelligence Committee with a new revelation about Trump's efforts to pressure Kiev to investigate the Democrat Biden -- the matter at the heart of just the fourth impeachment process in US history.

Trump is accused by Democrats of abusing his power by using US military assistance and a possible White House meeting to pressure President Volodymyr Zelensky into opening a probe into Biden and his son Hunter, who served on the board of Ukrainian gas company Burisma.

The key evidence is the official White House transcript of a July 25 telephone call between Trump and Zelensky in which the US president asked his Ukrainian counterpart to open investigations into the Bidens.

Taylor testified that he was told Trump cared more about the probe into his rival than he did about Ukraine.

The grey-haired former US Army officer and veteran diplomat, who testified in a closed hearing last month, said he had since become aware of a telephone call between Trump and Gordon Sondland, the US ambassador to the EU, that was overheard by a member of Taylor's staff.

Taylor said the staff member asked Sondland after the call what Trump thought about Ukraine.

"Ambassador Sondland responded that President Trump cared more about the investigations of Biden," Taylor said.

Asked about the new allegations, as he hosted his Turkish counterpart Recep Tayyip Erdogan at the White House, Trump replied: "First time I've heard it."

"He did speak to me for a brief moment," he said of Sondland, "and I said no quid pro quo under any circumstances."

'He didn't open investigations'

Republican lawmakers sought to undercut the testimony of the witnesses by focusing on Hunter Biden's role on the Burisma board, pointing out that he was paid $50,000 a month and questioning his qualifications.

They also stressed that the Ukrainians were not aware for months that the White House had put a hold on the nearly $400 million in military assistance and that it was eventually released in September.

"What did President Zelensky actually do to get the aid?" asked John Ratcliffe, a Republican congressman from Texas. "The answer is nothing. He didn't open investigations.

"He didn't do any of the things that House Democrats say that he was being forced and coerced and threatened to do," Ratcliffe said.

In his opening statement, Taylor recalled his opposition to making US military aid to Ukraine or a White House visit by Zelensky contingent on Kiev investigating the Bidens.

"Withholding security assistance in exchange for help with a domestic political campaign in the United States would be crazy," he said. "I believed that then and I believe it now."

Taylor said an "irregular policy channel" involving former New York mayor Rudy Giuliani, Trump's personal attorney, was pushing for the Ukrainian probe into the Bidens.

George Kent, a bow-tie wearing career diplomat who was the other witness at the somber and occasionally acrimonious hearing, was asked what interests Giuliani was promoting.

"I believe he was looking to dig up political dirt against a potential rival in the next election cycle," said Kent, a deputy assistant secretary of state.

"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."

'Scorched-earth war'

Democrats who control the House plan to prove over several weeks of hearings that Trump abused his office by asking Ukraine to investigate the Bidens.

If the House impeaches the president, it would then go for a trial in the Senate, where Republicans enjoy a 53-47 majority.

The next hearing is scheduled for Friday featuring the US ambassador to Ukraine who Kent said was recalled by Trump after being subjected to a "smear campaign" by Giuliani.

Eight more witnesses, including Sondland, are to appear next week.

The investigation threatens to make Trump only the third US president to be impeached, after Andrew Johnson in 1868 and Bill Clinton in 1998, although to be removed from office he would need to be convicted by the Senate.

Neither Johnson or Clinton was convicted and removed. But in 1974 Richard Nixon resigned in the face of certain impeachment and removal from office for the Watergate scandal.

Speaking before the hearing, Nancy Pelosi, the Democratic speaker of the House, said the probe was necessary to show Trump he is "not above the law.

"And that he will be held accountable."

Opening the proceedings, committee chairman Adam Schiff, the California congressman overseeing the probe, said it sought to establish "whether President Trump sought to exploit (Ukraine's) vulnerability and invite Ukraine's interference in our elections."

"If this is not impeachable conduct, what is?"

Devin Nunes, the top Republican on the panel, hit back by accusing Democrats of a "carefully orchestrated media smear campaign."

"It's nothing more than an impeachment process in search of a crime."

Coming just one year before elections, the hearings carry great risks for both parties and no certain reward, with a divided US electorate weary of Washington infighting.

Polls show a slim majority of Americans favor impeaching the president.

But they also show that Trump's sizable voter base, which delivered his shock victory in 2016, rejects the allegations. (AFP)