President Donald Trump at the Eisenhower Executive Office Building on the White House complex in Washington. File picture: Manuel Balce Ceneta/AP

Washington - John Bolton, former national security advisor to US President Donald Trump, said Monday that he would comply with a subpoena to testify in a Senate impeachment trial.

"I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify," Bolton said in a statement on his website.

The announcement is a major boost to US Democrats who have called for key administration witnesses, including Bolton and acting White House chief of staff Mick Mulvaney, to be summoned to testify in the imminent impeachment trial in the Republican-controlled Senate.

Bolton's name came up repeatedly during the impeachment investigation against Trump. Witnesses said he was aware of, and opposed to, the backdoor pressure channel on Ukraine at the heart of the investigation.

Bolton, who left his post in September, would be the highest-ranking White House official to provide testimony about Trump's efforts to pressure Ukraine to investigate Democratic political rival Joe Biden while withholding nearly 400 million dollars in military aid to Kiev.

Former White House official Fiona Hill testified before the House that Bolton called the backdoor channel a "drug deal" and told her to notify lawyers for the Nationa Security Council.

Hill also said Bolton called Trump's personal lawyer Rudy Giuliani - who was leading the backdoor channel - a "hand grenade that is going to blow everybody up."