At least one week before President Trump spoke by phone with newly-elected Ukrainian President Volodymyr Zelensky in late July, he instructed his acting chief of staff, Mick Mulvaney, to hold off on releasing nearly $400 million in military aid for Ukraine that had already been appropriated by Congress, CBS News has confirmed. A senior administration official with direct knowledge of the Trump administration's actions regarding the funds confirmed the delay in military aid.

Mr. Trump has admitted that during the call, he talked about former Vice President Joe Biden and his son, Hunter, who had accepted a seat on the board of directors of a Ukrainian natural gas company.

In mid-July, Office of Management and Budget (OMB) officials informed the State Department and Pentagon of the president's wish to delay the Ukrainian funding, the senior administration official also confirmed.

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But White House spokesperson Hogan Gidley told CBS News, "The media pushed the Russia lie for almost 3 years with no evidence, and now they are doing it all over again. These allegations are completely false, but because the media wants this story to be true so badly, they'll once again manufacture a frenzy and drive ignorant, fake stories to attack this president."

The Washington Post, which first reported the timing of the decision to withhold the Ukrainian aid, also reported that the OMB officials told the State Department and Pentagon that Mr. Trump "had 'concerns' and wanted to analyze whether the money needed to be spent."

Ultimately, the White House released the funds to Ukraine in September, after withholding the aid for about two months.

The Ukraine call is the subject of a whistleblower complaint from the intelligence community that was determined by the community's inspector general to be both credible and of "urgent concern." The complaint was supposed to be turned over to Congress — and House Intelligence Chairman Adam Schiff has subpoenaed it — but the acting director of national intelligence, Joseph Maguire, is refusing to submit it to the House panel. He has said the complaint wasn't of urgent concern.

Congressional Democrats have voiced suspicions that Mr. Trump may have been threatening to withhold the funding from Ukraine at the same time he was urging its new president to investigate the Bidens. Mr. Trump denies it. "I didn't do it at all," he told reporters Monday.

Seven moderate freshman House Democrats wrote in an opinion piece Tuesday that if Mr. Trump did pressure Zelensky to investigate Biden, he should be impeached.

"If these allegations are true, we believe these actions represent an impeachable offense," Representatives Gil Cisneros of California, Jason Crow of Colorado, Chrissy Houlahan of Pennsylvania, Elaine Luria of Virginia, Mikie Sherrill of New Jersey, Elissa Slotkin of Michigan and Abigail Spanberger of Virginia wrote Monday in the Washington Post.

House Speaker Nancy Pelosi is scheduled to meet with key committee leaders Tuesday afternoon to discuss the whistleblower complaint and other matters.

The Washington Post reports that Pelosi "has been quietly sounding out top allies and lawmakers about whether the time has come" to impeach Mr. Trump.

"She was making calls as late as Monday night, and many leadership aides who once thought Trump's impeachment was unlikely now say they think it's almost inevitable," the Post said.

-- Rebecca Kaplan contributed to this report