Democrats have renewed their demand for US President Donald Trump’s former National Security Adviser John Bolton to be called to testify in the president’s impeachment trial.
It follows Mr Bolton reportedly claiming that Mr Trump wanted to freeze aid to Ukraine unless it investigated his rival, Democrat Joe Biden.
The claims, apparently made in a book manuscript, are central to the trial due to resume later on Monday.
Mr Trump denies the charges.
But they could damage his defence.
Mr Trump told reporters last week that he did not want Mr Bolton to testify, citing national security concerns.
The president is alleged to have withheld military aid to pressure Ukrainian President Volodymyr Zelensky into starting a corruption investigation into Mr Biden, and his son Hunter.
President Trump has repeatedly branded the impeachment proceedings a “witch hunt”.
What do the new reports allege?
On Sunday, the New York Times cited excerpts from an unpublished book by Mr Bolton, who was fired from the White House in early September.
They included claims that Mr Trump told Mr Bolton in August that he wanted to withhold $391 million security aid to Ukraine until officials there assisted with probes into Democrats, including Mr Biden.
If true, such a claim would undermine the main argument of Mr Trump’s defence team that there was no “quid pro quo” which offered aid in return for investigating Mr Biden.
“Bolton directly contradicts the heart of the President’s defence,” House impeachment manager Adam Schiff tweeted.
“If the trial is to be fair, Senators must insist that Mr Bolton be called as a witness, and provide his notes and other documents.”
House Speaker Nancy Pelosi said that Republicans’ refusal to call Mr Bolton and others is “now even more indefensible”.
But Mr Trump denied the apparent claim, writing on Twitter: “I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens.
“In fact, he never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book.”
Earlier Mr Bolton’s lawyers said it was clear that the book’s “pre-publication review process has been corruption and that information has been disclosed by persons other than those properly involved in reviewing the manuscript”.
According to the New York Times, they blamed the White House for the leak, saying it had received a copy to review it for any disclosure of classified information.
The latest reports come after a video made public on Saturday showed Mr Trump ordering the removal of the US ambassador to Ukraine in 2018.
With Republicans holding a 53-47 majority in the Senate, it is unlikely that the president will be removed from office after being impeached by the House of Representatives.
He is the third president in US history to face an impeachment trial.