US President Donald Trump's former national security adviser, John Bolton, said on Monday that he is willing to testify in the expected Senate impeachment trial of the president in a surprise development that could complicate a weeks-long dispute over how the trial would play out.

"If the Senate issues a subpoena for my testimony, I am prepared to testify," Bolton said in a prepared statement. He declined further comment.

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Democrats have been pressing for Bolton and three senior administration officials to testify as part of the Senate trial. Trump's fellow Republicans have held firm that any decision on witness testimony at the Senate trial must wait until after opening statements are heard.

I have posted a brief statement regarding testimony on the Ukraine impeachment matter before the Senate at: https://t.co/Q3TwI2BzBp — John Bolton (@AmbJohnBolton) January 6, 2020

"John Bolton should testify," Mark Warner, the top Democrat on the Senate intelligence committee, said on Twitter.

House of Representatives Speaker Nancy Pelosi - the top Democrat in Congress - has yet to send the House-approved articles of impeachment to the Senate, putting a question mark over the schedule for a trial and buying time for Democrats to try to build the case for witnesses.

Senate Republican leader Mitch McConnell, whom Pelosi accuses of working to protect the president, has said a trial cannot begin until the charges are formally sent to the chamber, although another senior Republican called on Sunday for the rules to be changed if Pelosi does not act soon.

"We're not going to let Nancy Pelosi use the rules of the Senate to her advantage," Senate Judiciary Committee Chairman Lindsey Graham, a Trump ally, told Fox News Channel.

Without the articles in hand, McConnell said on Friday that senators would focus on "ordinary business" - in Monday's case, a nomination for a new head of the Small Business Administration.

The earliest the House could take any action would be on Tuesday when it reconvenes, but top Democrats have given no sign they are set to move this week.

"I don't think it's going to be indefinite," House Intelligence Committee Chairman Adam Schiff, a Democrat, told CNN on Sunday when asked about the delay in transmitting the impeachment articles.

The Democratic-led House has charged Trump with abusing his power for personal gain by asking Ukraine to announce a corruption investigation of former Vice President Joe Biden, a leading contender for the Democratic nomination to face Trump in November's presidential election.

It also charged the president with obstructing Congress by directing administration officials and agencies not to cooperate with the impeachment inquiry.

Trump says he did nothing wrong and has dismissed his impeachment as a partisan bid to undo his 2016 election win.

The Senate, which is controlled by Republicans 53-47, is unlikely to vote to find the president guilty and remove him from office, an act that would take a two-thirds majority.

Democrats have been hopeful they could persuade a few Republicans to side with them on their push for witnesses, which would require only a simple majority and could unearth evidence damaging to Trump.