President Donald Trump instructed a top White House official to withhold roughly $400 million in military aid to Ukraine shortly before his July 25 call with its newly elected president, The Washington Post reported on Monday.

Trump reportedly told the acting White House chief of staff, Mick Mulvaney, to hold the funds, which the US had provided in light of Ukraine's conflicts with pro-Russian separatists.

Officials informed congressional leaders that the two-month pause was due to an "interagency process," The Post reported.

Trump has denied holding up the funds to pressure President Volodymyr Zelensky's government to investigate former US Vice President Joe Biden. But he appeared to acknowledge on Monday that the aid package was on his mind during the July 25 phone call.

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President Donald Trump instructed a top White House official to withhold roughly $400 million in military aid to Ukraine shortly before his July 25 phone call with its newly elected president, according to a Washington Post report on Monday.

Trump told the acting White House chief of staff, Mick Mulvaney, to hold the funds — which the US had provided in light of Ukraine's conflicts with pro-Russian separatists — for at least a week before the call in July, three officials told The Post.

The funds, meant to provide Ukraine with military weapons, were eventually released September 11.

Officials from the Office of Management and Budget, which Mulvaney still leads, told the US State Department and the Defense Department that Trump had "concerns" over the funds, The Post reported. They reportedly informed congressional leaders that the two-month pause was due to an "interagency process."

The delay in sending the aid package has attracted scrutiny amid reports of a whistleblower complaint lodged against Trump that is thought to center on a July 25 phone call he had with Ukrainian President Volodymyr Zelensky. The complaint was said to involve a "promise," though it's unclear what the promise was or even whether it was directly related to the call.

The Wall Street Journal reported last week that during the phone call, Trump pressed Zelensky at least eight times to work with Trump's personal lawyer, Rudy Giuliani, to investigate former Vice President Joe Biden and Biden's son Hunter over their dealings in Ukraine.

The younger Biden sat on the board of Burisma Holdings, a Ukrainian gas-extraction company, from 2014 to early this year. Trump and Giuliani accused the elder Biden of trying to stymie a criminal investigation into Burisma in 2016 by pushing the Ukrainian government to fire Viktor Shokin, the prosecutor general leading the inquiry.

While Biden did seek the prosecutor's removal, those accusations are unsubstantiated, with government officials and Ukrainian anticorruption advocates in fact saying that Shokin had hampered the investigation into Burisma long before Biden had entered the picture, according to The Journal.

In other words, Biden was trying to oust a prosecutor who was slow-walking the investigation into Burisma, rather than actively targeting the company.