Pressure has grown for former National Security Adviser John Bolton to testify in US President Donald Trump's impeachment trial.
Calls for his testimony come amid claims that the adviser directly linked Mr Trump to a plan to freeze aid to Ukraine in exchange for a political favour.
The claims, reportedly made in a book manuscript, are central to the trial that has resumed on Monday.
Mr Trump denies the charges.
But the claims allegedly made by Mr Bolton could damage the president's defence.
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