WASHINGTON - 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".(FA)