The Democrats on Sunday seized on a newly released email on military aid to push for more testimony in the impeachment of United States President Donald Trump, as the White House signalled it was comfortable with plans by Senate Republicans to avoid calling any new witnesses.

The internal email, part of a series published by the investigative non-profit Center for Public Integrity, revealed a budget official told the Pentagon to “hold off” on military aid to Ukraine 91 minutes after a controversial phone call between Trump and his Ukrainian counterpart Volodymyr Zelenskyy.

Trump is accused of withholding $400m in assistance to Ukraine to push Kyiv to investigate his political rival Joe Biden.

“Based on guidance I have received and in light of the Administration’s plan to review assistance to Ukraine … please hold off on any additional DoD obligations of these funds,” Office of Management and Budget official Michael Duffey wrote in an email to Pentagon officials.

The email was time-stamped 11:04am – an hour and 31 minutes after Trump’s controversial July 25 phone call with Zelenskyy ended, according to a summary of the conversation released by the White House.

“Given the sensitive nature of the request, I appreciate your keeping that information closely held to those who need to know,” Duffey added.

Republicans defended the move in a December 2 House of Representatives staff report, saying it was “not unusual” for foreign aid to be delayed, the Center for Public Integrity noted.

The aid and the Trump-Zelenskyy call are at the heart of the impeachment case and Duffey is one of four witnesses top Democrat Chuck Schumer has proposed calling to give evidence.

Republican Senator Ron Johnson on Sunday told ABC’s This Week that the “new emails don’t shed any new light” on Trump’s rationale for withholding aid to Ukraine.

“The president was concerned about whether or not America’s hard-earned taxpayer dollars should be spent into a country where there’s been proven cases of corruption,” he said.

Schumer, in contrast, called the emails “explosive” in a tweet on Sunday and denounced Trump’s refusal to let certain White House officials testify.

“If nothing is wrong with withholding the aid, why didn’t Michael Duffey want anyone to know about what he was doing?” Schumer wrote.

From whom did Michael Duffey get the order and why did he do it? Was it directly from President Trump? Was it Mick Mulvaney? Until we hear from him and the witnesses we requested, Americans will be left to assume those blocking their testimony are aiding & abetting a cover-up. pic.twitter.com/4ajmBwftYp — Chuck Schumer (@SenSchumer) December 22, 2019

Trump was impeached by the House of Representatives for abuse of power and obstruction of Congress on December 18.

Despite testimony from 17 officials that Trump leveraged his office for political gain, the president has maintained his innocence denouncing the inquiry as an “attempted coup” and an “assault on America.”

He is only the third US president to be impeached and faces trial in the Senate, possibly in January.

Legislators left Washington, DC on Friday for the winter break at loggerheads over how to proceed.

Nancy Pelosi, the top Democrat in the House of Representatives has postponed sending the charges to the upper house, while Senate Majority Leader Mitch McConnell has said he wants the Senate to consider the case without hearing from new witnesses.

Marc Short, chief of staff to Vice President Mike Pence, indicated the White House backed McConnell.

“To the extent that there’s a prolonged trial, we’re not anxious about that,” Short told NBC’s Meet the Press. “Our administration is anxious to get back to working for the American people. .. We’ve had a lot of witnesses already.”