US President Donald Trump speaks to reporters before leaving the White House in Washington D.C. Sept. 12, 2019. (Photo by Ting Shen/Xinhua)

A top White House official admitted Thursday that President Donald Trump had tied military aid for Ukraine to Kiev undertaking an investigation that would help Trump politically, adding ammunition to Democrats' impeachment probe.

Acting White House Chief of Staff Mick Mulvaney's stunning admission supports a key allegation in the House impeachment effort, that the president illicitly sought foreign political help as he seeks re-election next year.

Mulvaney said it was true that Trump used the aid to prod Kiev in July to investigate the allegation - widely viewed as groundless - that the Democratic National Committee (DNC) had hidden a computer server in Ukraine that would undermine claims that Trump's 2016 election victory was helped by Russian meddling.

Trump did not want to send nearly $400 million in military aid to Ukraine because the country was deeply corrupt, Mulvaney said.

"Did he also mention to me in passing the corruption related to the DNC server? Absolutely. No question about that," Mulvaney told reporters.

"That's it, and that's why we held up the money."

Hours later, Mulvaney abruptly recanted.

"Let me be clear, there was absolutely no quid pro quo between Ukrainian military aidand any investigation into the 2016 election," he said in a statement.

"The president never told me to withhold any money until the Ukrainians did anything related to the server."

Nevertheless, his initial admission appeared to support Democrats' charges that Trump abused his power and violated election laws.

But Mulvaney argued that Trump had done nothing wrong, because he acted in support of a Justice Department probe into the origins of 2016 Russian meddling allegations. And Mulvaney denied that Trump also tied the aid to Ukraine investigating a top Democratic rival, Joe Biden.