US Secretary of State Mike Pompeo has confirmed he was listening in as the president asked Ukraine to investigate a political opponent.
The call - between Donald Trump and President Volodymyr Zelensky - is at the centre of a Democratic impeachment inquiry against Donald Trump.
Mr Pompeo dodged questions about the call in an interview just days ago.
President Trump denies improperly pressuring Ukraine and has accused opponents of mounting a "coup".
During the conversation, which triggered a whistleblower's formal complaint, the US president asked Mr Zelensky to investigate Democratic rival Joe Biden and his son Hunter.
Mr Pompeo's admission during a news conference in Italy came after media reported he was party to the conversation as well. "I was on the phone call," Mr Pompeo said.
He did not directly answer a question on whether anything in the phone call alarmed him, but said it had focused on US policy with Ukraine.
"It's been remarkably consistent, and we will continue to try to drive those set of outcomes," he said.
Mike Pompeo is walking a fine line.
On one hand, the former member of Congress wants to respect the legislative branch's investigatory prerogatives. He was, after all, right in the middle of the extensive questioning of Hillary Clinton as part of the 2015 Republican-led Benghazi hearings into the 2012 jihadist attack on the US consulate there while Mrs Clinton led the state department.
On the other, the secretary of state is in the executive branch now. He answers to Donald Trump, whose administration has fought a pitched battle to resist nearly every congressional request for documents and testimony.
Hence Mr Pompeo's sternly worded rebuke of House Democrats in a Tuesday letter, followed by Wednesday's assurance that "we will of course do our constitutional duty to co-operate with this co-equal branch".
Complicating all of this is the fact that Mr Pompeo, due to his participation in Mr Trump's July Ukraine call and evasive response to subsequent press questions about his involvement, is right in the middle of the investigation - making him more than simply a disinterested party.
Mr Pompeo may attempt to justify the president's attempts to pressure the Ukrainian president to investigate Joe Biden as "fully consistent with US policy", but Democrats - and a big chunk of the American public - don't see it that way.
On Tuesday, Mr Pompeo wrote to Democrats leading the impeachment effort to remove Mr Trump from office to say that he objected to their effort to "bully" five former and current state department officials who they wish to interview.
In other developments:
- Russia's President Vladimir Putin has said he always assumes phone calls with foreign leaders could be made public. CNN reported the White House had tried to restrict access to such conversations
- The state department's Inspector General has requested "urgent" meetings today with congressional committees to brief them on Trump White House interactions with Ukraine
- Mr Trump's personal lawyer Rudy Giuliani told Fox News he wants to sue Democrats in the House of Representatives for "interfering" with the president's business
- In a tweet, Mr Trump said the impeachment effort to remove him from office is a "coup, intended to take away the Power of the People"
Mr Pompeo's presence on the call was reported earlier this week, but it is the first time he has addressed it directly.
When asked in a 22 September interview with ABC News what he knew of the conversation, Mr Pompeo said he had just been given the "whistle-blower complaint, none of which I've seen".
"You just gave me a report... none of which I've seen," Secretary of State Mike Pompeo tells @martharaddatz when asked about a report that President Trump pressed the President of Ukraine to work with Rudy Giuliani to investigate Joe Biden's son. https://t.co/CaWsz4XapY pic.twitter.com/coDl6mG6Dy— ABC News (@ABC) September 22, 2019
Pressed on whether Mr Trump's reported comments were appropriate, he responded: "I think I saw a statement from the Ukrainian foreign minister yesterday, said there was no pressure applied in the course of the conversation."
Mr Pompeo - the top US diplomat - was hit last week with a House subpoena to testify. On Tuesday three Democratic committee chairmen accused him of "stonewalling" the impeachment inquiry, adding that he is a "fact witness" to the probe.
His three day trip to Italy, which includes a private audience with Pope Francis, has largely been overshadowed by the impeachment efforts in Washington.
Quick facts on impeachment
- Impeachment is the first part - the charges - of a two-stage political process by which Congress can remove a president from office
- If the House of Representatives votes to pass articles of impeachment, the Senate is forced to hold a trial
- A Senate vote requires a two-thirds majority to convict - unlikely in this case, given that Mr Trump's party controls the chamber
- Only two US presidents in history - Bill Clinton and Andrew Johnson - have been impeached but neither was convicted and removed
- President Nixon resigned before he could be impeached