President Donald Trump's personal lawyer, Rudy Giuliani, has said removing the US ambassador to Ukraine became imperative for him and Mr Trump.
The ex-US envoy, Marie Yovanovitch, fired abruptly in May, has been a key witness in impeachment proceedings against Mr Trump.
"I needed Yovanovitch out of the way," Mr Giuliani told the New Yorker.
The House of Representatives votes this week on impeachment, a probe centred on Mr Trump's Ukraine dealings.
Mr Trump is accused of pressuring Ukraine's leader to dig up supposedly damaging information on a former Vice-President Joe Biden, a domestic political rival. The Republican president has denied all wrongdoing, dismissing the inquiry as a "witch hunt".
The House Rules Committee is meeting to determine how the House debate should be conducted. Mr Trump faces two charges - abuse of power and obstruction of Congress.
The House debate, with votes expected on Wednesday or Thursday, will mark the third time in history that the lower chamber has discussed impeaching a president.
Why is Ms Yovanovitch important?
Ms Yovanovitch's sudden dismissal has emerged as a key element of the inquiry. Speaking before the House Intelligence Committee last month, the 33-year veteran of the foreign service said she believed her anti-corruption efforts in Ukraine had incurred the ire of influential Ukrainians who sought to remove her.
The ambassador said she was shocked that her enemies appeared to find allies in the Trump administration, including Mr Giuliani.
Mr Giuliani's interview with the New Yorker appears to confirm that Ms Yovanovitch's dismissal was linked to investigations into Ukraine sought by Mr Trump.
The former New York mayor has become a central character in the Ukrainian affair and the Democrat-led impeachment inquiry. Testifying last month, the US ambassador to the EU Gordon Sondland said that Mr Trump directed him and other top diplomats to work with Mr Giuliani on Ukraine "at the express direction of the president".
Ms Yovanovitch "was going to make the investigations difficult for everybody," Mr Giuliani told the New Yorker.
On Monday evening, he appeared to reinforce this stance, telling Fox News's Laura Ingraham that "of course" he had Ms Yovanovitch removed, claiming it was due to the diplomat's misbehaviour.
There is no evidence of any wrongdoing by Ms Yovanovitch.
Speaking to the New York Times in comments published on Tuesday, Mr Giuliani said he provided Mr Trump with detailed information about his probes into Ukraine, and how Ms Yovanovitch was frustrating his efforts.
Mr Giuliani visited Ukraine again last month, before meeting Mr Trump at the White House on Friday.
Asked about Mr Giuliani's work in Ukraine, the president told reporters his lawyer "does it out of love".
"He's a great person who loves our country," Mr Trump said. "He sees what goes on. He sees what's happening."
What else happened on Tuesday?
In a six-page letter to the Democratic leadership, the president accused them of "subverting American democracy".
He repeated his claim that the phone call with the Ukrainian president, a call which is at the centre of the impeachment inquiry, was "totally innocent".
"Any member of Congress who votes in support of impeachment - against every shred of truth, fact, evidence and legal principle - is showing how deeply they revile the voters and how truly they detest America's constitutional order.
"Our founders feared the tribalisation of partisan politics and you are bringing their worst fears to life."
He ends the letter by saying that 100 years from now, people will understand and learn from this, "so that it can never happen to another president again".
Want to find out more?
- A SIMPLE GUIDE: If you want a basic take, this one's for you
- GO DEEPER: Here's a 100, 300 and 800-word summary of the story
- WHAT'S IMPEACHMENT? A political process to remove a president
- VIEW FROM TRUMP COUNTRY: Hear from residents of a West Virginia town
- CONTEXT: Why Ukraine matters to the US
- FACT-CHECK: Did Ukraine interfere in the 2016 election to help Clinton?
- CASE FOR & AGAINST: What legal scholars say about Trump conduct