Trump call details were 'often concealed'

Details of US President Donald Trump's calls with foreign leaders were severely restricted after embarrassing leaks of his early conversations, a former White House official says.

The White House's handling of Trump's calls with foreign leaders is at the heart of Democrats' impeachment inquiry.

A whistleblower alleges the White House tried to "lock down" 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.

The former White House official acknowledged 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 Trump's presidency he was particularly enraged by leaks that disclosed tough conversations with the leader of Mexico on paying for a border wall and with then Australian prime minister Malcolm Turnbull on abiding by an Obama administration deal on asylum seekers.

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

Former administrations had kept the details 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.