Washington (CNN) A White House review of President Donald Trump's decision to withhold nearly $400 million in US military aid to Ukraine showed an extensive effort to justify that hold after it was placed and a debate over its legality, The Washington Post reported Sunday.

The White House counsel's office has been conducting an internal review of the aid freeze and circumstances surrounding Trump's July 25 call with his Ukrainian counterpart prompted by the House impeachment inquiry into Trump's dealings with Ukraine.

The Post, citing three people familiar with the records, said the office's confidential review surfaced hundreds of documents, including emails between acting White House chief of staff Mick Mulvaney and White House budget officials. Those emails from early August showed a search for an explanation for why Trump stalled the money Congress had approved to go to Ukraine after the President had ordered it held the month before, the Post said.

Officials close to Mulvaney have previously expressed frustrations about not being in the loop on the review. A senior administration official told CNN the chief of staff's office had not yet reviewed any of the findings of the internal review and had not been provided with the emails referenced in the Post story.

The delayed US aid, along with a whistleblower report on Trump's July 25 call, sits at the center of the Democrat-led impeachment probe into the President. House Democrats have argued that Trump used the delay of assistance as leverage to pressure Ukraine to investigate his political rivals on that call. The President has denied there was any "quid pro quo" and Republicans have argued that bribery could not exist if Ukraine was not aware that the assistance was being held up.

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