Senate Minority Leader Chuck Schumer continued to push for top administration officials to appear before the Senate in the upcoming impeachment trial, citing a newly revealed, "explosive" email from a White House aide as evidence for why witnesses should be called.

"Yesterday, we got new evidence about how important having these witnesses [is]," Schumer, a Democrat, said at a press conference Sunday in New York.

A new email obtained by the Center for Public Integrity and released Saturday shows Michael Duffey, associate director for National Security Programs at the Office of Management and Budget (OMB), informed the Pentagon of the freeze on $391 million in security assistance to Ukraine just 91 minutes after President Trump's call to Ukrainian President Volodymyr Zelensky on July 25.

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During the call, Mr. Trump urged Zelensky to open an investigation into his Democratic political rivals. The phone call prompted House Democrats' impeachment inquiry into the president, as they examined whether Mr. Trump conditioned the security assistance and a White House meeting on a public announcement from Zelensky of investigations into former Vice President Joe Biden and his son Hunter.

Senate Minority Leader Chuck Schumer holds a press conference in New York on Sunday, December 22, 2019. CBS News

In the email, which was obtained through a Freedom of Information Act request, Duffey directed the Pentagon to "please hold off on any additional DoD obligations of these funds" based on guidance he received and "in light of the administration's plan to review" the aid to Ukraine.

"Given the sensitive nature of the request, I appreciate your keeping that information closely held to those who need to know to execute the direction," Duffey wrote in the email dated July 25.

Duffey is one of four White House officials that Schumer has called to testify before senators during the upper chamber's impeachment trial. Senate Majority Leader Mitch McConnell, however, has rebuffed the request. Duffey did not appear before the House during its months-long impeachment inquiry, as the White House blocked aides from complying with congressional subpoenas for testimony and agencies from turning over documents.

Schumer and McConnell are at an impasse over the structure of the Senate trial. Schumer said he and McConnell had a "cordial" meeting on Thursday but remained in disagreement over whether to call witnesses. The Democratic leader said he asked McConnell to reconsider his position over the holidays.

In a statement on Sunday, OMB spokeswoman Rachel Semmel said it would be "reckless" to suggest the hold on money was connected to Mr. Trump's phone call with Zelensky. The military aid was ultimately released in September.

"As has been established and publicly reported, the hold was announced in an interagency meeting on July 18," Semmel said. "To pull a line out of one email and fail to address the context is misleading and inaccurate."

But Schumer called the content of Duffey's email "rather explosive" and questioned what the White House was hiding in requesting the hold on assistance be kept quiet.

"So far, Senator McConnell and President Trump have come up with no good reason why there shouldn't be witnesses, why there shouldn't be documents," he said. "We don't know what the witnesses will say. We don't know how the documents will read. They might exonerate President Trump or they might further incriminate him. But the truth should come out on something as important as an impeachment."

The House on Wednesday passed two articles of impeachment charging Mr. Trump with abuse of power and obstruction of Congress. But House Speaker Nancy Pelosi has not yet sent the articles to the Senate, as she has expressed concerns the upper chamber will not hold a fair trial.

Sara Cook contributed reporting.