Details of Donald Trump's phone calls with multiple foreign leaders have been hidden, a White House official said.

According to the anonymous whistleblower, Trump's calls with Vladimir Putin and Saudi Arabia's Mohammed bin Salman were among the calls which were filed as severely restricted memos.

It comes as Democrats seek to impeach Mr Trump over allegations he asked Ukraine's new leader to investigate presidential hopeful Joe Biden.

A whistleblower claimed the White House tried to "lock down" Mr Trump's call with Volodymyr Zelensky because they were worried about the request to investigate Biden.

The White House has also allegedly tried to cover up content of other calls by moving memos onto a highly classified computer system, the anonymous source added.

The former staffer admitted other calls were concealed, while casting the decision as part of an effort to minimise leaks, not an attempt to conceal information.

The early part of Trump's presidency was hit with leaks over phone conversations the US president had with Mexico's leader about paying for the border wall and Australia on abiding to an Obama administration deal on asylum seekers.

Previous administrations have kept rough transcripts of calls but they have not been kept on a highly classified computer system unless they contained details relating to national security.

Summaries of the calls were distributed to relevant officials in the White House, the State Department and other agencies.

The Trump administration’s process curtailed the number of people who had access.

A rough transcript of Mr Trump's phone conversation with the Ukraine leader were released by the White House this week, although the contents of the calls with bin Salman and Putin remain unknown.

Mr Trump's relationship with both leaders has been controversial, in part due to Russia's interference in the US presidential election, and Saudi Arabia's human rights violations and the murder of Jamal Khashoggi.

Mr Trump has repeatedly tried to discredit the whistleblower, accusing the anonymous CIA officer of having ulterior political motives.

His advisers, however, have confirmed some details of the whistleblower’s complaint, but offered different explanations for the White House’s actions.

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