U.S. President Donald Trump's former national-security adviser, John Bolton, says he is willing to testify at the expected impeachment trial of the president.
"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.
As a top White House aide, Bolton's firsthand knowledge of what transpired in events that prompted the House of Representatives to impeach Trump in December could provide new evidence of the president's efforts to prod Ukraine to investigate a political rival.
However, since the Senate is controlled by Republicans aligned with Trump, it is unlikely Bolton will be called as a witness.
Congressional investigators believe Bolton likened the White House's decision to delay $390 million in military aid to Ukraine to a "drug deal," according to witness testimony during the House's fact-finding phase of the impeachment proceedings.
So far, the Democratic-led House hasn't sent the articles of impeachment, or formal charges, to the Senate for a trial.
Both sides have yet to reach an agreement on how the trial should proceed.
Republicans control the Senate by a margin of 53-47.
Trump has been charged with abusing the power of his office for personal gain and obstructing Congress by withholding requested documents and blocking the testimony of administration officials and agencies during the impeachment inquiry.