An inquiry looking at whether US President Donald Trump should be impeached has held its first public hearing.

He is accused of abusing power by withholding aid for Ukraine unless the country investigated his political rival, Joe Biden.

Mr Trump denies this.

He also said earlier on Wednesday that he would not be watching the televised proceedings.

"I'm too busy to watch it - it's a witch-hunt, it's a hoax. I'm too busy to watch it, so I'm sure I'll get a report".

Asked by a reporter if he had been briefed on the hearing, he said: "I have not been briefed, no".

William Taylor, the top US diplomat in Ukraine, has given evidence to the inquiry.

He spoke about a member of his staff who heard Mr Trump asking another senior diplomat about the investigations he wanted Ukraine to pursue.

"Following the call with President Trump, the member of my staff asked Ambassador Sondland what President Trump thought about Ukraine.

"Ambassador Sondland responded that President Trump cares more about the investigations of Biden, which Giuliani was pressing for."

"At the time I gave my deposition on October 22nd, I was not aware of this information - I'm including it here for completeness.

"As the committee knows, I reported this information through counsel to the State Department's legal advisor - as well as to counsel for both the majority and the minority on the committee.

"It is my understanding that the committee is following up on this matter."

Mr Taylor added: "I recognise that this is a rather lengthy recitation of the events of the past few months, told from my vantage point in Kyiv.

"But I also recognise the importance of the matters your committee is investigating, and I hope that this chronology will provide some framework for your questions.

US President Donald Trump in the Oval Office with Republican Senators at the White House in Washington DC | Image: Patrick Semansky/AP/Press Association Images

"As I mentioned in my October 22nd deposition, the information and quotes in my testimony are based on my best recollection as well as a review of my personal notes."

The inquiry stems from a phone call between Mr Trump and Ukrainian President Volodymyr Zelenskiy on July 25th.

A White House memorandum suggests Mr Trump urged Mr Zelenskiy to work with his lawyer Rudy Giuliani and the US attorney-general to investigate Joe Biden.

In the call, Mr Trump raised unsubstantiated allegations that Mr Biden, the former US vice president and Democrat candidate in the 2020 election, sought to interfere with a Ukrainian prosecutor's investigation of his son Hunter.

"There's a lot of talk about Biden's son, that Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the Attorney General would be great," Mr Trump said Mr Zelenskiy.

"Biden went around bragging that he stopped the prosecution so if you can look into it ... It sounds horrible to me."

Some Republicans have implied that US officials attempted to block investigations into Hunter Biden sitting on the board of Ukrainian company, Burisma.

But another witness to the inquiry on Wednesday, Deputy Assistant Secretary of State George Kent, said he had not heard any US official attempt to stop investigations.

Mr Kent said he had raised concerns about the issue in 2015, saying it could give the "perception of a conflict of interest".

But he did not go into detail about Mr Trump's conversation with Mr Zelensky.

Top US Democrat, House Speaker Nancy Pelosi, announced in September that the House of Representatives would examine whether Mr Trump sought Ukraine's help to smear Mr Biden - the frontrunner for the 2020 Democratic presidential nomination.

Ms Pelosi said such actions would mark a "betrayal of his oath of office". She declared: "No one is above the law."