Ukraine's former Foreign Minister Pavlo Klimkin has categorically rejected claims by Donald Trump concerning Mr Trump's Democratic rival Joe Biden.
Mr Trump has alleged, without evidence, that Mr Biden pressed for the sacking of a Ukrainian prosecutor to protect a business that employed his son.
Mr Klimkin told the BBC that the prosecutor was sacked for corruption.
A number of Western bodies, including the EU, had pushed for the prosecutor, Viktor Shokin, to be sacked, he said.
Mr Trump faces impeachment proceedings for using his position as president to push the Ukrainian President, Volodymyr Zelensky, to investigate Mr Biden, who was at the time leading polls to be his Democratic opponent in the 2020 election.
The Democrats issued their first Subpoena of the impeachment inquiry on Friday - ordering Secretary of State Mike Pompeo to hand over documents by 4 October relating to the administration's dealings with the Ukrainian government
Earlier it was revealed by US media that the whistleblower whose complaint led to the impeachment inquiry was a CIA officer who once worked at the White House.
And 300 former US national security officials signed a letter supporting the impeachment. They described Mr Trump's actions as a "national security concern", and said he appeared to have committed "an unconscionable abuse of power".
Mr Trump's Democrat opponent in the 2016 elections, Hillary Clinton, also backed impeachment, saying the president "poses a clear and present danger to our future, to our democracy".
Mr Trump dismissed the impeachment proceedings as a "witch-hunt".
Impeachment is a two-stage political process, rarely exercised, by which a US president can be removed from office for wrongdoing.
Even if President Trump was impeached by the House of Representatives, he is unlikely to be forced out of the White House because Republicans control the Senate.
What did Mr Klimkin say?
Mr Klimkin, who was serving as foreign minister when Mr Shokin was sacked in 2016, said there was "definitely" no evidence that his removal was for anything other than corruption.
"The whole sense of this push was to sort out, to deliver on reforms in Ukraine," he said. "It was not about the prosecutor general. It was about prosecutor offices which were systemically corrupt."
He said it was important to have a full reshuffle of the offices, and pressure was coming from the whole international community, not just Mr Biden or the US individually.
The BBC's Jonah Fisher in Kiev says Mr Klimkin's words serve to reinforce what has been the almost universal reaction from credible voices in Kiev, namely that Mr Shokin had been an obstacle to fighting corruption.
What is the claim about Joe Biden?
On 25 July Mr Trump raised Mr Shokin's removal during a phone call with newly-elected Ukrainian President Volodymyr Zelensky, details of which were released by the White House this week.
He went on to discuss Mr Biden's son Hunter and the unsubstantiated allegation that his father - then the US vice-president - stopped an investigation into his son's employer by lobbying Ukraine to fire Mr Shokin.
The chief prosecutor's office had an open inquiry into Burisma, a natural gas company on which Hunter Biden was a board member.
There is no evidence of any wrongdoing by the Bidens.
Why is the phone call controversial?
Democrats accuse Mr Trump of illegally seeking foreign help in the hope of smearing Mr Biden - who is seeking the Democratic nomination for the 2020 presidential election.
The US president called on the Ukrainian leader to talk to US Attorney General William Barr and Mr Trump's lawyer Rudy Giuliani about investigating Hunter Biden's past business dealings.
Mr Trump is accused of using military aid to Ukraine as a bargaining tool. The package - which has since been released - was aimed at supporting US interests in a friendly country.
Mr Trump, a Republican, denies any wrongdoing and has dismissed the impeachment proceedings as a "hoax" and "another witch-hunt".
He acknowledged that he had personally blocked nearly $400m (£324m) in military aid to Ukraine days before he spoke to Mr Zelensky, but denied that it was to pressure the Ukrainian leader into investigating Mr Biden.
Who is the whistleblower and what do they say?
Few details are known. A lawyer for the whistleblower warned that trying to identify the person could place them "in harm's way".
The New York Times, Washington Post and Reuters news agency identified the whistleblower as a CIA officer.
Their complaint accuses Mr Trump of "using the power of his office to solicit interference from a foreign country in the US 2020 election".
The declassified document characterises the president's conduct as a "serious or flagrant problem, abuse, or violation of law".
The whistleblower says they had learned from several sources that senior White House officials had intervened to "lock down" all records of the call, particularly an official word-for-word transcript.
"This set of actions underscored to me that White House officials understood the gravity of what had transpired in the call," the whistleblower wrote in the complaint.
How the controversy unfolded
- Mid-July - President Trump orders White House aide to hold back almost $400m in military aid to Ukraine, report US media
- 25 July - Mr Trump speaks to Ukraine's leader in a 30-minute phone call
- 9 September - Congress learns of a whistleblower's complaint about the call, but is blocked by the Trump administration from viewing it
- 11 September - Military aid for Ukraine is cleared for release by the Pentagon and Department of State
- 23 September - Mr Trump confirms he withheld Ukrainian aid, saying it was due to concerns about "corruption"
- 24 September - Mr Trump says the aid was withheld so that other countries would pay more.