The acting White House chief of staff, Mick Mulvaney, has reversed comments he made regarding the Trump administration's dealings with Ukraine.
Mulvaney said on Thursday that the White House had never required Ukraine to investigate the 2016 US election in exchange for military aid.
"Let me be clear, there was absolutely no quid pro quo between Ukrainian military aid and any investigation into the 2016 election" Mulvaney said.
This was in contrast to his comments to reporters earlier in the day, that there was "no question" the request to investigate a Democratic National Committee server was one of the reasons the White House delayed providing Ukraine with the funds.
The senior White House official had acknowledged that President Donald Trump held up $US391 million ($A573 million) in military aid to Ukraine in part to pressure the Ukrainians to look into an allegation about the 2016 US presidential election that has been debunked as a conspiracy theory.
Trump and administration officials have denied for weeks that the President demanded a favour for delivering the US aid, a key part of the controversy that has triggered an impeachment inquiry.
In a July 25 call, Trump asked Ukrainian President Volodymyr Zelenskiy to look into the server and another matter relating to the 2016 election.
Trump asked Zelenskiy to investigate his political opponent, Joe Biden, and his son Hunter Biden, who had served as a director for a Ukrainian energy company.
Zelenskiy agreed during the call to carry out the investigation Trump sought. The US aid later was provided to Ukraine.
Meanwhile the US ambassador to the European Union said in written testimony to the impeachment inquiry that Trump told senior US officials to talk directly to his personal lawyer, Rudy Giuliani, about US policy in Ukraine, raising concern the president was outsourcing US foreign policy to a private citizen.
The envoy, Gordon Sondland, told the Democratic-led inquiry that he did not understand "until much later" that Giuliani's agenda included a push for Ukraine to investigate Biden.
"Please know that I would not have recommended that Mr. Giuliani or any private citizen be involved in these foreign policy matters," Sondland said, but added that given Trump's "explicit direction" that "we agreed to do as President Trump directed."