Ukrainian ex-Foreign Minister Pavlo Klimkin has rejected claims by US President Donald Trump over the sacking of ex-chief prosecutor Viktor Shokin.
Mr Trump has alleged, without offering evidence, that the sacking was over an investigation into a firm where ex-Vice-President Joe Biden's son worked.
Mr Klimkin told the BBC Mr Shokin was sacked due to systemic corruption.
Mr Trump faces impeachment proceedings for urging Ukraine to probe Mr Biden, his main rival for the presidency.
Meanwhile 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.
Three-hundred former US national security officials have signed a letter supporting the impeachment.
They describe Mr Trump's action as a "national security concern", and say he appears to have committed "an unconscionable abuse of power".
Mr Trump has 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.
When Mr Shokin was replaced, his successor continued to investigate Burisma for 10 months before the inquiry ended.
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 also accused of using military aid to Ukraine as a bargaining tool. The package – which has since been released – had been appropriated by Congress to support 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?
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.
Meanwhile, an audio recording has emerged in which Mr Trump demands to know who provided information to the whistleblower.
"I want to know who's the person, who's the person who gave the whistleblower the information. Because that's close to a spy," the pRead More – Source