WASHINGTON (NYTIMES) - John Bolton, the former White House national security adviser, said on Monday (Jan 6) that he was willing to testify at United States President Donald Trump's impeachment trial if he was subpoenaed.

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

The development is a dramatic turn in the impeachment proceeding, which has been stalled over Democrats' insistence on hearing from critical witnesses Mr Trump blocked from testifying in the House inquiry into his pressure campaign on Ukraine.

Mr Bolton is a potential bombshell of a witness, with crucial knowledge of the president's actions and conversations regarding Ukraine that could fill in key blanks in the narrative of the impeachment case.

His willingness to tell the Senate what he knows ratchets up pressure on Senator Mitch McConnell, the majority leader, who has refused to commit to calling witnesses at the impeachment trial, to change his stance.

It is unclear how the White House will respond to Mr Bolton's declaration.

"It now falls to the Senate to fulfil its constitutional obligation to try impeachments, and it does not appear possible that a final judicial resolution of the still-unanswered constitutional questions can be obtained before the Senate acts," Mr Bolton wrote.

"Accordingly, 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."

If he does appear under oath in the Senate, Mr Bolton would be the closest adviser to the president to testify about what Mr Trump said behind closed doors as he pressured the Ukranians to investigate his political rivals as he withheld nearly US$400 million (S$540 million) in military aid from the country.

The Democratic-led House impeached Mr Trump last month on charges of abuse of power and obstruction of Congress, charging him with a corrupt scheme to solicit help from Ukraine in the 2020 election and concealing his actions from Congress.

Mr Trump has repeatedly sought to block his most senior aides, as well as former advisers who have left the White House, from speaking to Congress, and has gone to court to stop several of them from cooperating.