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Former White House National Security Adviser John Bolton says he is willing to testify at President Donald Trump's impeachment trial in the Senate.
"I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify," Mr Bolton said in a statement.
He has previously complied with the White House directive not to co-operate with the Democratic-led inquiry.
Mr Bolton would be the most senior former Trump adviser to testify.
Mr Trump, a Republican, was impeached by the Democratic-controlled House of Representatives last month. He was the third US president ever to incur such a congressional sanction.
Mr Bolton wrote on his website on Monday: "During the present impeachment controversy, I have tried to meet my obligations both as a citizen and as former National Security Adviser.
"Since my testimony is once again at issue, I have had to resolve the serious competing issues as best I could, based on careful consideration and study."
The hawkish former national security adviser was fired by President Trump in September after 16 months on the job.
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Don't hold your breath
John Bolton is a potentially important witness in the abuse-of-power case against Donald Trump. If he testifies in a Senate trial of the president, it could be a blockbuster moment.
It's hard not to see Mr Bolton's latest announcement, however, in the context of his previous hints, pirouettes and provocative tweets on testifying.
While he threatened a protracted legal battle to prevent appearing before House impeachment investigators, for instance, his lawyer hinted that there were many previously undisclosed "meetings and conversations" of which Mr Bolton had knowledge.
Stonewall, wink, repeat.
It may be a good strategy for staying in the public eye - and, perhaps, boosting upcoming book sales - but Democrats will find it infuriating.
It's also far from guaranteed that Mr Bolton will ever be subpoenaed by the Senate.
His announcement might marginally increase the pressure on moderate Senate Republicans to join with Democrats and form a majority to call for witnesses in the presidential trial.
Republican Senate Leader Mitch McConnell has proven skilled at keeping his party in line, however, and immune to pressure when it comes to pursuing the course of action he views as most politically beneficial.
It will take more than a Bolton statement to make him fold.
Why might Bolton's testimony be important?
His potential firsthand knowledge of key events surrounding the Democratic-led investigation would make him a prize witness.
Mr Trump is accused of abusing the power of his office by withholding US military aid in order to pressure Ukraine into smearing his political rival, former US Vice-President Joe Biden.
Mr Bolton likened that alleged White House bargaining chip to a "drug deal", according to witness testimony during the House impeachment investigation.
His aides testified that he was deeply concerned by Mr Trump's actions related to Ukraine.
Mr Bolton has kept a relatively low profile since leaving the White House, though he inked a book deal in November with publishing giant Simon & Schuster, reportedly worth $2m (£1.5m).
When is the Senate trial?
Amid a political row, House Democrats have held off formally sending the two articles of impeachment to the Senate, which would trigger a trial in that chamber. It is so far unclear when they might do so.
The Republican-controlled Senate has been resisting Democratic demands for witnesses to be called in the trial. Mr Bolton's announcement may put pressure on the chamber's Republican leader, Mitch McConnell, to allow such testimony.
Speaking from the chamber floor on Monday, Senate Democratic leader Chuck Schumer compared Mr McConnell's handling of the impeachment proceedings to "Alice in Wonderland - first the trial and then the evidence".
"If the Senate were to agree to leader McConnell's proposal, the Senate would act as little more than a nationally televised meeting of a mock trial club," said the New York Democrat.
The Senate is controlled by the president's fellow Republicans and therefore it is highly unlikely they will vote for Mr Trump to be removed from office.
John Bolton Trump Impeachment Trial