Robert Mueller's probe implicated Donald Trump in acts of obstruction of justice. (File)

Long-silent Special Counsel Robert Mueller will deliver his first public statement on the Russia meddling investigation on Wednesday, as he faces pressure to testify in Congress on his explosive findings.

Mueller, whose probe implicated President Donald Trump in acts of obstruction of justice, will make the statement at the Justice Department at 11 am (1500 GMT) but take no questions, the department said.

Democrats in Congress have been pressing Mueller for weeks to testify on the findings of his report, completed in late March following a high-stakes investigation that loomed large throughout the first two years of Trump's presidency.

The report described extensive contacts and possible attempted collusion, but no criminal conspiracy between Trump's 2016 election campaign and Russia.

It also mapped out 10 instances of possible obstruction of justice by Trump, but refused to rule whether it was enough for criminal prosecution.

Democrats considering impeaching the president have demanded Mueller, a former FBI director, testify on his findings in Congress.

He has reportedly offered to speak in private, but the Democrat-led House Judiciary Committee wants him to testify publicly, and has suggested Mueller could be subpoenaed to speak.

Trump and the Justice Department head, Attorney General Bill Barr, say Mueller's report clears Trump of wrongdoing, and appear opposed to Mueller being questioned on the report on Capitol Hill.

"The Dems want a second shot at Bob Mueller, are very unhappy with the No Collusion Report," Trump tweeted last week.

"They should not be allowed to play this game any longer - no second chances - must get back to work. So bad for our Country!"

Ever since Mueller was named as an independent prosecutor to lead the Russia investigation in May 2017, Trump has branded it a "witch hunt" and a "hoax".

Trump claimed "complete exoneration"

Mueller though produced criminal charges against 37 individuals and companies, including six former Trump associates, including campaign chairman Paul Manafort, personal lawyer Michael Cohen, and national security advisor Michael Flynn.

Mueller's final report was clouded by huge controversy over its findings on Trump.

Barr released his own summary weeks before the full report was out, declaring it showed no evidence of collusion with Russia and showed "no actions that, in our judgment, constitute obstructive conduct."

Trump seized on that to declare that it was a "complete exoneration."

In a letter to Barr released later, however, Mueller took issue with his conclusions, saying that they did not correctly capture the substance of the report and his conclusions.

Democrats have since demanded to see the entire report without redactions as well as the underlying evidence, and have subpoenaed Trump's former White House lawyer Don McGahn, Mueller's primary source on many of the obstruction allegations, to testify.

The White House has refused to permit McGahn to testify, setting up a court battle and fuelling calls by Democrat legislators to launch impeachment proceedings against Trump.

Mueller's testimony would be important in any such proceedings.