Democratic lawmakers said on Wednesday Mueller's testimony on July 17 will refute descriptions of his report put forward by President Donald Trump and his Attorney General William Barr.

"I think it will have a profound impact," said House of Representatives Judiciary Committee chairman Jerrold Nadler, one of a handful of senior Democrats leading investigations of Trump, his turbulent presidency and his business interests.

"Attorney General Barr led a campaign of misinformation and deceived the American people about what was in the report. The president chimed in by repeatedly saying 'no collusion', which is not what the report says," Nadler said.

Mueller's 448-page report, released publicly in April, found Russia meddled in the 2016 presidential election and that President Donald Trump's election campaign had multiple contacts with Russian officials. But it found insufficient evidence to establish a criminal conspiracy between the campaign and Moscow.

Trump has said the report shows he did not collude with Russia. The White House had no comment on Nadler's remarks.

Republicans said Mueller's appearance before Nadler's panel and the House of Representatives Intelligence Committee will give them a chance to advance their inquiry into the origins of the Russia probe, which they contend was part of an effort by politicised federal officials to undermine Trump.

"There's just a plethora of questions here that all go back to the timing, the integrity and the heart of this report. And those are all questions now that he's either going to have to answer or he's going to have to sit there and try to avoid," Representative Doug Collins, the top Republican on Nadler's committee, told Fox News.

The Mueller probe, originally launched by the FBI, has clouded Trump's presidency since he entered office in January 2017 and the administration's response has been to try to undermine Mueller and the FBI, while stonewalling Democrats in congress who took up the probe after Mueller completed his work.

Earlier on Wednesday, Trump himself accused Mueller, without citing any evidence, of illegally terminating communications between two former FBI employees early in the investigation.

"Mueller terminated them illegally. He terminated all of the emails ... Robert Mueller terminated their text messages together. He terminated them. They're gone. And that's illegal. That's a crime," Trump said in an interview with Fox Business Network, referring to pair of FBI employees who exchanged disparaging messages about him.

Democrats said the White House could try to prevent Mueller from testifying about his report but that it would likely fail.

"Mr Mueller is an honest man who understands that congressional subpoenas are not optional," Nadler said.

Australian Associated Press