US Democrats will open a formal impeachment hearing into President Donald Trump over claims that he sought political help from Ukraine.
House Speaker Nancy Pelosi announced announced the inquiry after a closed-door meeting with Democratic lawmakers.
It follows Mr Trump acknowledging withholding US aid to Ukraine and pressuring it to investigate Joe Biden, who is in the running to be the Democratic candidate for the presidency.
Mr Trump denied using the funds as political leverage, insisting he only wanted Europe to step up assistance to the Eastern European country.
Ms Pelosi said at a media conference this morning Mr Trump's actions appeared to have undermined national security and violated the US Constitution.
"The president must be held accountable. No one is above the law," Ms Pelosi said.
An attempt to remove Mr Trump from office would require about 20 Republicans in the Senate to rebel against him.
No US president has ever been removed from office by impeachment.
While in New York City for the United Nations General Assembly on Tuesday, Mr Trump tweeted he had authorised the release "of the complete, fully declassified transcript of my phone conversation with President Zelensky of Ukraine".
"You will see it was a very friendly and totally appropriate call," he wrote.
On Tuesday, Mr Trump said he only froze military aid to Ukraine because he wanted European countries to contribute funds too.
"We're putting up the bulk of the money, and I'm asking why is that?" he said. "What I want, and I insist on it, is that Europe has to put up money for Ukraine also."
The Republican president also acknowledged pressuring newly elected Ukrainian President Volodymyr Zelensky during a phone call on 25 July to investigate US Democratic presidential frontrunner Joe Biden.
"That call was perfect, it couldn't have been nicer," said Mr Trump, who is up for election next year. "There was no pressure put on them whatsoever.
"But there was pressure put on with respect to Joe Biden. What Joe Biden did for his son, that's something they should be looking at."
Later on Tuesday, Mr Trump told reporters: "Ask how his [Joe Biden's] son made millions of dollars from Ukraine... even though he had no expertise whatsoever, okay."
Mr Trump and his conservative allies have pointed out that Joe Biden, while US vice-president, threatened in 2016 to withhold aid to Ukraine unless it fired a top prosecutor whose office had been investigating a natural gas company where Hunter Biden was a board member.
Other Western officials had called for the same prosecutor to be removed on the grounds that he was soft on corruption. Ukraine's current prosecutor, Yuriy Lutsenko, told Bloomberg News in May he had no evidence of wrongdoing by Mr Biden or his son.
Mr Trump's latest remarks came after it emerged that days before his phone call with Mr Zelensky, the US president had instructed his acting chief of staff, Mick Mulvaney, to withhold nearly $US400m in military aid for Ukraine.
Congressional Democrats have accused Mr Trump of trying to enlist a foreign power to smear a domestic opponent.
They are demanding a transcript of the phone call, which the White House declined to release.
The controversy came to light after a US intelligence whistleblower filed a complaint with an internal watchdog about Mr Trump's conversation with the Ukrainian president.
How the impeachment process works:
- 1. Any member can introduce a resolution of impeachment if they suspect the president is guilty of "treason, bribery or other high crimes or misdemeanours"
- 2. The House of representatives considers the set of charges. If a simple majority (51%) support impeachment, the process moves to trial.
- 3. The trial begins. The chief justice of the Supreme Court presides over it while members of the House form the prosecution and Senators act as the jury. The President can appoint defence lawyers.
- 4. At the end of the trial, a Senate vote delivers the outcome. If at least two-thirds (67%) of Senators find the president guilty, he is removed from office and the vice president takes over for the remainder of the term.
On Monday night, the Washington Post published an op-ed by seven Democratic lawmakers - all with backgrounds in the US military and intelligence agencies - who said the "stunning" accusations against Mr Trump amounted to "a national security threat".
"If these allegations are true, we believe these actions represent an impeachable offence," wrote the lawmakers. "We do not arrive at this conclusion lightly."
Nearly a dozen Democrats have come out in favour of impeachment over the last week since the Ukraine phone call controversy came to light.
More than 145 House Democrats now back such a move - more than half of the party's 235 members in the lower chamber of Congress. The Senate, though, remains in Republican hands and is unlikely to back impeachment.
Mr Biden will make a statement on Tuesday afternoon about the president's "ongoing abuse of power", his campaign said.
Speaking in Wilmington, Delaware, he will also say Congress should impeach Mr Trump if he fails to comply with requests for information on Ukraine.
The Biden team has hit back aggressively at the Ukraine allegations, denying any wrongdoing.
Mr Biden told journalists on Saturday: "Trump's doing this because he knows I will beat him like a drum." He also raised his voice and jabbed his finger at a Fox News reporter, telling him: "Ask the right questions!"