Ex-US National Security Adviser John Bolton has been summoned to testify in the congressional impeachment inquiry into President Donald Trump.
House of Representatives investigators have invited him to appear next month.
Mr Trump has denied using the power of his office to pressure Ukraine to investigate former US Vice-President Joe Biden and his son.
Mr Bolton likened the alleged proposal to a “drug deal”, according to witness testimony.
It is unclear if Mr Bolton, who left his position in September after falling from favour with the president, will appear as scheduled on 7 November.
Mr Bolton was also alarmed about the interactions of President Trump’s private lawyer Rudy Giuliani with Ukraine, according to US media.
A Department of State diplomat reportedly told House impeachment investigators that Mr Bolton had warned him about Mr Giuliani’s influence with the president on Ukraine.
Mr Giuliani acknowledged last month in a TV interview having asked Ukraine to investigate Mr Biden.
The former US vice-president is a leading Democratic candidate to challenge Mr Trump for the White House next year.