Democrats have renewed their demand for US President Donald Trump’s former National Security Advisor John Bolton to be called to testify in the president’s impeachment trial.
The call comes following reports that Mr Bolton made claims in an unpublished book that Mr Trump wanted to freeze aid to Ukraine unless it investigated his political rival, Democrat Joe Biden.
It could undermine Mr Trump’s defence.
The impeachment trial will resume in the US Senate later on Monday.
Mr Trump told reporters last week that he did not want Mr Bolton to testify.
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”.