The newly-released email was obtained and made available on Sunday by the non-profit Center for Public Integrity following a court order in a freedom of information case, the BBC reported
It revealed that a senior White House official, Mike Duffey, contacted senior defence officials about withholding Ukraine's aid just over an hour-and-a-half after Trump ended the July 25 call with President Zelensky.
A rough transcript of that call was declassified by the White House following a whistleblower complaint it was being covered-up.
The transcript shows that Trump asked Zelensky to "do us a favour" and investigate Joe Biden, currently a frontrunner to be the Democratic candidate in the 2020 White House race, and his son Hunter Biden, who had previously worked for a Ukrainian energy company.
In the email, Duffey asks that the Department of Defense "hold off" on providing aid following the administration's plan to review.
"Given the sensitive nature of the request, I appreciate your keeping that information closely held to those who need to know to execute direction," the email reads.
The documents were obtained by the non-profit following a judge order and show the call between the rulers occurred between 9.03 a.m. and 9.33 a.m. on July 25, 91 minutes before Duffey's 11.04 a.m. email - which also appears in another batch at 3.03 p.m., Efe news reported.
This development comes after the Democrat-controlled House of Representatives voted on December 18 to pass two articles of impeachment against Trump, making him the third president to be indicted in the country's history.
Responding to the development, Rachel Semmel, a spokeswoman for the Office of Management and Budget, in a statement on Sunday said: "It's reckless to tie the hold of funds to the phone call. As has been established and publicly reported, the hold was announced in an interagency meeting on July 18.
"To pull a line out of one email and fail to address the context is misleading and inaccurate."
Meanwhile the top Democrat in the US Senate, Chuck Schumer, said the email proved the need for new witnesses and evidence at President Trump's impeachment trial, the BBC reported.
"If there was ever an argument that we need Duffey to come and testify, this is that information. This email is explosive," Schumer said.
The next step in Trump's impeachment following the House votes is a a trial in the Republican-controlled Senate next month.
But Nancy Pelosi, the Democratic Speaker of the House, has postponed sending the impeachment charges to the Senate until the rules of the Senate trial are acceptable to her party.
Trump tweeted on Sunday that the Democrats' case in the "impeachment hoax" was "dead" and called Pelosi "crazy Nancy".
Republicans currently hold 53 seats in the 100-seat chamber, making a conviction - which requires a two-thirds vote - extremely unlikely.
( With inputs from IANS )