US President Donald Trump says he is unconcerned about the impeachment inquiry into him but does not want to support it by letting his top aide testify.
Speaking at the White House, Trump dismissed US diplomats' recent release of transcripts to Democrat-led House of Representatives committees conducting the investigation.
The committees are looking into whether the Republican president pressured Ukraine to probe his domestic political rivals.
Trump also criticised Democrats for moving their inquiry into the public eye with open, televised hearings next week.
"They shouldn't be having public hearings; this is a hoax," Trump, who has denied any wrongdoing, told reporters on Friday.
He accused Democrats of looking for people who hated him to testify in the probe and said he was not familiar with most of the witnesses, who include a number of top US State Department officials.
"I'm not concerned about anything. The testimony has all been fine. I mean, for the most part, I never even heard of these people. I have no idea who they are," he said.
House investigators are still pursuing testimony from witnesses behind closed doors, including from acting White House chief of staff Mick Mulvaney.
Mulvaney was subpoenaed on Thursday night to appear but did not show up on Friday.
His lawyers told investigators Mulvaney had been directed by the White House not to comply with the subpoena and asserted 'absolute immunity', a congressional aide said.
Mark Sandy, associate director for national security programs at the White House's Office of Management and Budget (OMB), also was called to testify and did not show up.
The White House previously has said it would not co-operate with the congressional investigation, which was triggered by a whistleblower complaint about Trump's July 25 telephone call with Ukrainian President Volodymyr Zelenskiy.
Lawmakers wanted to question officials about their knowledge of OMB's decision last summer to block, without explanation to Congress, nearly $US400 million ($A584 million) in security aid for Ukraine that had been approved by lawmakers.
