Donald Trump repeatedly urged Ukraine's new leader to investigate his political rival Joe Biden, a memo of a telephone call shows.
In the call, the US president raised unsubstantiated allegations that former vice president Mr Biden sought to interfere with a Ukrainian prosecutor's investigation into his son Hunter, a lawyer and lobbyist.
Speaking to Ukraine's Volodymyr Zelenskiy, Mr Trump said: "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."
In the days before the phone call, Mr Trump ordered advisers to freeze $400m (£323m) in military aid for Ukraine, prompting speculation that he was holding out the money as leverage for information on the Bidens.
A whistleblower complained about a 30-minute conversation between the two leaders which took place on 25 July.
In his first solo news conference since the proceedings were brought against him, Mr Trump defended the call, saying it was "perfect".
Mr Trump went on to say the phone call was the second with Mr Zelenskiy and that if the press keep asking for it, he will release the first phone conversation.
He said he "fully supports transparency" but expects scrutiny of Mr Biden. The president tweeted to say he "also insist[s] on transparency from Joe Biden and his son Hunter".
Earlier in the day, at a joint news conference between the leaders of both countries in New York, the Ukrainian president echoed Mr Trump's denials by insisting he was not pressured by his American counterpart - and attempted to limit his involvement in the row.
Mr Zelenskiy said: "I think you read everything. I am sorry, but I don't want to be involved in [the] democratic elections of USA.
"We had a good phone call. It was normal. We spoke about many things... nobody pushed it."
Mr Trump is under pressure to release the complaint and a full transcript of the telephone call.
The complaint and Mr Trump's then-refusal to provide information prompted top Democrat Nancy Pelosi on Tuesday to officially launch an impeachment inquiry .
She announced the House of Representatives will examine whether Mr Trump sought Ukraine's help to smear Mr Biden, the frontrunner for the 2020 Democratic presidential nomination.
Mrs Pelosi, the House speaker, said such actions would mark a "betrayal of his oath of office" - declaring: "No one is above the law."
Speaking to reporters early on Wednesday, Mr Trump said "there was no pressure whatsoever" on Mr Zelenskiy.
The president acknowledged temporarily blocking the funds but said he had done nothing wrong and was subject to a "witch hunt".
Mr Trump said: "Just so you understand, it's the single greatest witch hunt in American history. Probably in history. But in American history. It's a disgraceful thing.
"The way you had that built up - that call was going to be the call from hell. It turned out to be a nothing call. Other than that a lot of people said I never knew you could be so nice."
Wednesday's news conference with Mr Zelenskiy got off to a light-hearted start, with the Ukrainian leader joking that he preferred being on TV "than on the phone". However, there was a tense finish following questions from reporters.
Mr Trump accused Mrs Pelosi of having "lost her way" and being "taken over by the radical left".
He then said "as far as I'm concerned she's no longer speaker of the House" - despite the fact she still holds that position.
Reacting to the rough transcript in the memo, house intelligence chairman Adam Schiff said of Mr Trump: "This is how a Mafia boss talks."
The Democrat told reporters on Wednesday that he was "shocked" by the conversation, adding: "It is far more damning than I had imagined."
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Mr Schiff released a joint statement with Jerrold Nadler, the chairman of the house judiciary committee, Elijah E Cummings, the chairman of the committee on oversight and reform, and Eliot L Engel, the chairman of the committee on foreign affairs.
In it, they said: "The transcript is an unambiguous, damning, and shocking abuse of the office of the presidency for personal political gain.
"This is a clear breach of trust placed in the president to faithfully execute the laws and to preserve, protect, and defend the Constitution.
"The record of the call released by the White House confirms our worst fears: that the president abused his office by directly and repeatedly asking a foreign country to investigate his political rival and open investigations meant to help the president politically.
"Not once, not twice, but more than half a dozen times during one telephone call. This was a shakedown."
In the memo, Mr Trump said he would get former mayor of New York City, Rudy Giuliani, who is his personal lawyer, and attorney general William Barr to call Mr Zelenskiy.
By attempting to get American law enforcement directly involved, Mr Trump personally attempted to use the government to act on a politically motivated investigation. This is the first time there has been evidence of such behaviour.
The US president said in the call there was "a lot of talk about Biden's son".
Mr Trump claimed the former vice president wielded his influence to benefit his son's private-sector work in Ukraine.
He said: "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.
"Biden went around bragging that he stopped the prosecution so if you·can look into it... it sounds horrible to me."
The criticism being levelled at the president is not solely coming from Democrats.
Utah Senator Mitt Romney said he found the call between the two leaders "deeply troubling".
The whistleblower, a member of the intelligence community, said in their complaint that they had heard the information from "White House officials", though did not have first-hand knowledge of the call, a Justice Department official said.
Prosecutors from the department determined Mr Trump did not violate campaign finance law after reviewing the transcript, the official said.