Former United States national security adviser John Bolton was "part of many relevant meetings and conversations" pertaining to the House impeachment inquiry of President Donald Trump that are not yet public, his lawyer said.

Charles Cooper made the revelation in a letter that suggests Bolton will appear before Congress only if a judge orders him to do so.

The letter, addressed to the top lawyer for the House of Representatives, seeks to distinguish Bolton and former deputy Charles Kupperman from other current and former White House officials who have testified so far to impeachment investigators.

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The letter says that Bolton and Kupperman, unlike the other witnesses, provided direct advice to Trump regularly and would be asked during any congressional appearance to disclose sensitive foreign policy and national security information.

"After all, Dr Kupperman was the Deputy National Security Advisor to the President throughout the period to your inquiry," the letter states.

"The same is true, of course, of Ambassador Bolton, who was the National Security Advisor to the President, and who was personally involved in many of the events, meetings, and conversations about which you have already received testimony, as well as many relevant meetings and conversations that have not yet been discussed in the testimonies thus far."

Kupperman was subpoenaed as part of the impeachment inquiry but sued House Democrats and the Trump administration. He asked a judge to decide which directive he must follow - one from Congress ordering him to testify, the other from the White House telling him not to. Bolton has not been subpoenaed. Lawmakers scheduled a Thursday interview with Bolton, but he did not show.

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Cooper represents both Bolton and Kupperman.

"As I emphasised in my previous responses to letters from the House Chairs, Dr. Kupperman stands ready, as does Ambassador Bolton, to testify if the Judiciary resolves the conflict in favour of the Legislative Branch's position respecting such testimony," Cooper wrote.

Though he hasn't testified, Bolton has nonetheless been a key player during weeks of testimony, with multiple witnesses describing episodes and conversations he was involved in.

In a transcript of a closed-door interview released Friday, for instance, a former national security official described how Bolton had "immediately stiffened" as Ambassador Gordon Sondland "blurted out" that he had worked out a trade - Ukrainians' probe for an Oval Office welcome - with Trump's acting chief of staff, Mick Mulvaney.

Fiona Hill said Bolton later told her that "I am not part of whatever drug deal Sondland and Mulvaney are cooking up" and asked her to relay that message to a White House lawyer.