A senior White House official told the Pentagon to “hold off” on Ukrainian aid just 91 minutes after Donald Trump asked the country’s president to investigate Joe Biden.

The emails, obtained by The Centre for Public Integrity, shows Michael Duffey, a member of the Office of Management and Budget (OMB), told the US Department of Defence to wait because a review of aid was planned.

The email stated: “Based on guidance I have received and in light of the Administration’s plan to review assistance to Ukraine, including the Ukraine Security Assistance Initiative (USAI), please hold off on any additional DoD obligations of these funds.”

Stopping the aid may have broken a law called the Impoundment Control Act, which was designed to stop presidents ignoring the will of the Congress.

It would mean an attempt to prevent approved aid reaching Ukraine could be illegal, because Congress would have to approve any changes in how funding was spent.

In the email Mr Duffey also acknowledged the “sensitive nature” of the request and asked that only those who needed to know were told about the direction.

It wasn’t until 11 September that Mr Duffey told Elaine McCusker, principal deputy under secretary at the DoD, that he would be signing off an apportionment that would release all the funds for the USAI.

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He told her he was “glad to have this behind us”.

Another email reveals no one appeared to know any reason for the “hold off” on funding, with a budget official telling a defence official “I still have no insight on the rationale for the hold”.

The Centre for Public Integrity reports there was never a formal announcement of any changes to aid spending, nor was Congress informed.

The first emails came shortly after Mr Trump had called Ukrainian President Volodymyr Zelenskiy and asked him to investigate Mr Biden and his son Hunter Biden, who used to work for an energy company in the country.

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Mr Trump was impeached for abuse of power and obstruction of justice over the phone call.

During the call, Mr Zelenskiy said he wanted to continue military cooperation with the US and said his country was almost ready to buy more anti-tank missiles.

Ukraine is in a key position in the struggle against Russian aggression.

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Mr Trump had responded to Mr Zelenskiy by requesting two investigations – one into his opponent in 2016, Hillary Clinton, and another was into the apparently lucrative ties Hunter Biden had with Ukraine.

The US president has always denied anything was wrong with the calls, which came to light after a whistleblower raised concerns, with the president saying the conversations they had were “perfect”.

He is likely to be acquitted in a Senate trial as it would need two-thirds majority to carry and the chamber is currently controlled by the Republicans.

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In a statement to NBC News early Sunday the OMB sought to play down the significance of the new emails.

“It’s reckless to tie the hold of funds to the phone call,” said spokeswoman Rachel Semmel.

“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.”

The Pentagon did not immediately respond to a request for comment.