US President Donald Trump’s embarrassing leaked phone call with then-Prime Minister Malcolm Turnbull led to a regime of severe restrictions on access to transcripts, a former White House official says.

The admission could worsen Mr Trump’s impending impeachment battle over the White House’s handling of the President’s calls with foreign leaders.

A recent call with the leader of Ukraine is at the heart of the Democrats’ impeachment inquiry, that seeks to remove the President from office.

A whistleblower alleges the White House tried to “lock down” Mr Trump’s July 25 phone call with Ukraine’s new president because officials were worried about his request for help investigating Democratic rival Joe Biden.

The anonymous whistleblower alleges the White House also tried to cover up the content of other calls by moving memos onto a highly classified computer system.

A former White House official told the Associated Press that other calls were concealed, while casting the decision as part of an effort to minimise leaks, not an attempt to hide improper discussions.

In the early days of Mr Trump’s presidency he was particularly enraged by leaks that disclosed tough conversations with Mr Turnbull on abiding by an Obama administration deal on asylum seekers and with the leader of Mexico on paying for a border wall.

The Trump administration curtailed the number of people who had access to phone call transcripts in contrast to previous administrations.

Mr Turnbull told Mr Trump that Australia would take any refugee the US nominated – including “not very attractive” ones – if the President honoured the asylum seeker relocation deal brokered with Barack Obama, according to the sensationally leaked transcript of their infamous 2017 first phone call.

The classified transcript of the conversation between the two leaders was published by The Washington Post, along with the contents of another call between Mr Trump and the Mexican President Peña Nieto.

“Putin was a pleasant call,” Mr Trump told Mr Turnbull during the conversation. “This is ridiculous.”

Former administrations had kept the details of calls private, but not on the highly classified computer system unless sensitive national security information was discussed.

Meanwhile, a former US ambassador to NATO caught in the middle of the whistleblower complaint resigned from his post as special envoy to Ukraine on Saturday.

The move followed disclosures that Kurt Volker had connected Trump’s personal lawyer Rudy Giuliani with Ukrainian officials to investigate Biden and his family over allegedly corrupt business dealings.

The White House acknowledged on Friday that the Ukraine call was moved to a highly classified system at the direction of National Security Council lawyers.

Describing the Turnbull leak as “absolutely horrific”, Devin Nunes, the highest-ranking Republican member on the US House of Representatives Intelligence Committee, announced in April he was sending eight criminal referrals to the US Attorney General.

-with AAP