Mick Mulvaney, acting White House chief of staff, has acknowledged for the first time that U.S. aid to Ukraine was held up partly over President Donald Trump's concerns about a Democratic National Committee (DNC) computer server alleged to be in Ukraine.

The admission, which the White House later tried to row back, came as Trump announced another figure embroiled in the Ukraine scandal, Rick Perry, would resign as energy secretary.

Mulvaney's admission

Trump and administration officials had denied for weeks that they had demanded a "quid pro quo" for delivering the 391 million U.S. dollars in aid to Ukraine, a central factor in the impeachment inquiry in the House of Representatives against the Republican president.

"I have news for everybody: Get over it. There is going to be political influence in foreign policy," Mulvaney told reporters in a briefing on Thursday. He later contradicted himself, ruling out a quid pro quo in a statement from the White House.

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In a July 25 call, Trump asked Ukrainian President Volodymyr Zelensky for "a favor" to look into the DNC server as well as the California-based cybersecurity firm CrowdStrike, which was hired by the DNC to investigate hacking of Democratic emails.

Trump also asked Zelensky to investigate a domestic political opponent, Joe Biden, and Biden's son Hunter Biden, who had served as a director for a Ukrainian energy company. Zelensky agreed during the call to carry out the investigation that Trump sought. The U.S. aid was later released.

'Did he mention the server? Absolutely'

Mulvaney said Trump did not like foreign aid, thought Ukraine was corrupt and was annoyed at how little "lethal aid" European nations provided to Ukraine as it combated Russia-backed separatists in the eastern part of the country.

"Did he also mention to me in the past, the corruption related to the DNC server? Absolutely. No question," Mulvaney said, referring to Trump. "But that's it. That's why we held up the money."

"The look-back to what happened in 2016 certainly was part of the things that he was worried about in corruption in that nation," Mulvaney said, referring to Trump.

A reporter told the acting chief of staff that what he just described was a quid pro quo. "We do that all the time with foreign policy," Mulvaney responded. In his later statement, Mulvaney he insisted: "There was absolutely no quid pro quo.”

Perry quits

Energy Secretary Rick Perry will step down by the end of the year, Trump said on Thursday, a day before a deadline set by congressional Democrats for the former Texas governor to turn over documents in the impeachment probe.

Gordon Sondland, the U.S. ambassador to the European Union, earlier said in written testimony in the impeachment inquiry that Trump told senior U.S. officials – including Perry – to talk directly to his personal lawyer, Rudy Giuliani, about U.S. policy in Ukraine.

Mulvaney confirmed Trump directed Perry to work on Ukraine policy with Giuliani, raising concern that the president was outsourcing American foreign policy to a private citizen.

Sondland told lawmakers he did not understand "until much later" that Giuliani's agenda included a push for Ukraine to investigate Biden. More testimony is expected next week, including from a top Pentagon official who oversees policy on Ukraine, sources told Reuters.

(With input from Reuters)