WASHINGTON — US President Donald Trump on Wednesday ratcheted up attacks on his former National Security Adviser John Bolton, whose upcoming book could harm Trump's impeachment trial defense.

Bolton's book reportedly says that the president wanted to freeze military aid to Ukraine until Kiev opened an investigation into Trump's political rival Joe Biden.

Democrats want Bolton to testify at the president's ongoing Senate trial, and there have been growing indications that enough Republican senators could support the demand.

In an early-morning tweet, Trump again attacked Bolton — whom he fired in September — saying the hawkish former advisor had begged for the job and shown poor judgment.

Trump said Bolton "got fired because frankly, if I listened to him, we would be in World War Six by now."

"(He) goes out and IMMEDIATELY writes a nasty & untrue book. All Classified National Security. Who would do this?"

Calling Bolton, and potentially other witnesses, to testify could prolong the impeachment trial and torpedo efforts by the White House to bring proceedings to a close this week.

The accusation that funds were withheld from Ukraine for Trump's own political purposes was at the heart of his impeachment by the Democratic-controlled House of Representatives.

Four Republicans would need to join Democrats in the Senate, where Republicans hold a 53-47 edge, to allow testimony from Bolton and others.

Trump's former White House chief of staff, retired general John Kelly, on Tuesday threw further doubt on Trump's conduct.

"If John Bolton says that in the book, I believe John Bolton," Kelly said. — AFP