A second whistleblower has come forward with information about US President Donald Trump's dealings with Ukraine, a lawyer says.

Key points: Lawyer says the second whistleblower has been interviewed by the intelligence community's internal watchdog

Lawyer says the second whistleblower has been interviewed by the intelligence community's internal watchdog Second whistleblower hasn't filed an official complaint

Second whistleblower hasn't filed an official complaint Secretary of State says his department will follow the law in terms of the impeachment investigation

Mark Zaid, who represents the original whistleblower whose complaint has triggered the impeachment inquiry, said the second whistleblower also worked in intelligence and had "firsthand knowledge that supported" the original accusations.

The original whistleblower complained that Mr Trump was "using the power of his office to solicit interference from a foreign country" in the 2020 US election.

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

Mr Zaid said the second whistleblower had spoken to the intelligence community's internal watchdog, but had not filed a complaint with the inspector general.

Mr Trump had asked for an investigation into Joe Biden, his Democratic rival, in a July 25 phone call with new Ukrainian President Volodymyr Zelenskiy, at a time when millions of dollars in US military assistance to the country was being held up.

Mr Trump suggested Ukraine "look into" Mr Biden and his son Hunter. ( Reuters: Carlos Barria )

During the July phone call, Mr Trump said there had been "a lot of talk" that Mr Biden had shut down a Ukrainian investigation into a company where his son worked.

"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 in the call with President Zelenskiy, 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".

Mr Trump's allegations have largely been debunked and there has been no evidence of illegal conduct or wrongdoing in Ukraine by the Bidens.

The call occurred after Mr Trump had ordered the US Government to freeze about $391 million in American aid to Ukraine.

Mr Trump said he froze the aid to Ukraine because he wanted to "see other countries helping Ukraine also, not just us."

The US President ramped up his attack on the Bidens after the second whistleblower revelations came to light.

"The great scam is being revealed!" Mr Trump tweeted at one point.

The President also voiced his strong support among Republican voters and repeated a campaign slogan to "drain the swamp".

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Pelosi 'guilty of treason'

On Sunday (local time) Mr Trump also lashed out at US House of Representatives Speaker Nancy Pelosi, who last month announced the formal investigation that could lead to impeachment proceedings against the President.

"Nancy Pelosi knew of all of the many Shifty Adam Schiff lies and massive frauds perpetrated upon Congress and the American people," Mr Trump said on Twitter, referring to House Intelligence Committee chairman Adam Schiff.

Mr Schiff, who was tagged by Ms Pelosi to lead the impeachment investigation, said during a hearing last month that the phone call summary read like a "classic organised crime shakedown".

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And last week, a spokesman for Mr Schiff's committee acknowledged the whistleblower approached the panel for guidance before filing his complaint about Mr Trump's efforts to push Ukraine to pursue an investigation into the Bidens.

There is, however, no evidence Mr Schiff or the panel helped draft the complaint.

Mr Trump referred to both Mr Schiff's characterisation of his call with Mr Zelenskiy and the whistleblower's approach to the House Intelligence Committee in his latest attack on Ms Pelosi.

"This makes Nervous Nancy every bit as guilty as Liddle' Adam Schiff for High Crimes and Misdemeanors, and even Treason," he said on Twitter.

"I guess that means that they, along with all of those that evilly 'Colluded' with them, must all be immediately Impeached!"

State Department will comply with legal requirements, Pompeo says

Meanwhile, Congressional committees carrying out the impeachment investigation have ordered the White House to turn over documents to support the probe.

Secretary of State Mike Pompeo said the State Department had yet to turn over any documents, but intended to follow a proper review.

He said the department sent a letter to Congress on Friday night as its initial response to the document request.

"We'll obviously do all the things that we're required to do by law," Mr Pompeo said.

He is allowing Democrats to interview a series of department witnesses this coming week. Among them is Gordon Sondland, the US ambassador to the European Union, another key figure in the probe.

The Secretary of State has become a key figure in the Democrats' investigation.

He was on the line during the July phone call in which Mr Trump pressed Ukraine's president to investigate Mr Biden and his son Hunter.

Mr Trump recently told reporters that China should also investigate Mr Biden, who is a frontrunner in the 2020 US Presidential race.

The administration has struggled to come up with a unified response to the quickly progressing investigation. Democrats have warned that defying their demands will in itself be considered "evidence of obstruction" and a potentially impeachable offence.

AP