President Donald Trump's withholding of $391 million in military aid to Ukraine was linked to his request that the Ukrainians look into a claim — debunked as a conspiracy theory — about the 2016 U.S. election, a senior presidential aide said on Thursday, the first time the White House acknowledged such a connection.

Trump and administration officials had denied for weeks that they had demanded a "quid pro quo" — a Latin phrase meaning a favor for a favor — for delivering the U.S. aid, a key part of a controversy that has triggered an impeachment inquiry in the House of Representatives against the Republican president.

But Mick Mulvaney, acting White House chief of staff, acknowledged in a briefing with reporters that the U.S. aid — already approved by Congress — was held up partly over Trump's concerns about a Democratic National Committee (DNC) computer server alleged to be in Ukraine.