A second whistleblower with first-hand knowledge of US President Donald Trump’s dealings with Ukraine has come forward, a lawyer said.

Mark Zaid, a Washington-based lawyer who represents the original whistleblower whose complaint triggered an impeachment inquiry, announced on Twitter that a second person had “made a protected disclosure under the law and cannot be retaliated against”.

Mr Zaid confirmed in an interview with ABC’s This Week that the second official, also from the US intelligence community, has been interviewed by the inspector general.

He said the second whistleblower hasn’t filed an official complaint with the inspector general but does have “firsthand knowledge that supported” the original person.

Tweet from @MarkSZaidEsq

The first whistleblower filed a complaint involving a  phone call Mr Trump held in July with Ukraine’s President Volodymyr Zelenskiy, in which he asked him to investigate a Democratic rival, former vice president Joe Biden.

The request came at a time when millions of dollars in US military assistance to the country was being held up.

During the phone call, the President said there had been “a lot of talk” about whether Mr Biden shut down a Ukrainian investigation into the company his son worked for.

“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, according to the summary provided by the US Justice Department.

“Biden went around bragging that he stopped the prosecution so if you can look into it … It sounds horrible to me,” he said, according to the memo.

Mr Trump said the inquiry would be “a favour”.

There is no evidence of illegal conduct or wrongdoing by the Bidens in Ukraine.

The call prompted House of Representatives Speaker Nancy Pelosi to launch impeachment proceedings against Mr Trump, saying his actions jeopardised US election integrity and threatened national security.

On Friday, the New York Times reported that a second person was considering coming forward who had “more direct information” about the events surrounding the call.

It is not yet known if this is the person represented by Mr Zaid.

Mr Trump and his supporters have rejected the accusations that he did anything improper.

Mr Trump suggested Ukraine ‘look into’ Mr Biden and his son Hunter. Photo: Getty

Mr Trump’s personal lawyer Rudy Giuliani lashed out at the media tweeting that he was not surprised there was another “secret source”, saying the inquiry was politically motivated .

As news emerged on Saturday that a second whistleblower was preparing to step forward, Mr Trump also went on the attack.

“The first so-called second-hand information ‘Whistleblower’ got my phone conversation almost completely wrong,” the President tweeted.

“So now word is they are going to the bench and another ‘Whistleblower’ is coming in from the Deep State, also with second-hand info. Meet with Shifty. Keep them coming!”

“Shifty” is Mr Trump’s smear for Representative Adam Schiff, the chairman of the intelligence committee which is heading up the impeachment inquiry.

The existence of the second whistleblower followed stirrings of discontent within Mr Trump’s own Republican Party after he called on China to investigate Biden’s son, who had business dealings in China.

Republican US Senators Mitt Romney, Ben Sasse and Susan Collins all have expressed concerns about Trump reaching out to foreign countries to help him in his 2020 re-election bid.

-With AAP