United States Attorney General William Barr spent Saturday reviewing the special counsel’s confidential report on the Trump-Russia investigation with Washington anxious to learn if US President Donald Trump is implicated in serious wrongdoing.

Special counsel Robert Mueller on Friday submitted his long-awaited report into an explosive two-year investigation of Russian meddling in Trump's 2016 election — a probe the president denounces as a "witch hunt" and opponents say could fuel impeachment.

What the report says is confidential, but Attorney General Barr wrote in a letter to US Congress that he might be able to summarise its "principal conclusions" for Congress as early as this weekend.

Chronic tweeter Trump, who headed out from his Florida residence early for a round of golf, remained uncommonly silent — suggesting the White House has still not seen the report.

Meanwhile, Democrats in Congress, many of who are hoping for evidence to support a presidential impeachment, were preparing to meet as they pressed hard to ensure the report's full contents are made public, and not just a summary prepared by the Trump-appointed Barr.

"It is imperative for Mr Barr to make the full report public and provide its underlying documentation and findings to Congress," House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer said late Friday in a statement, adding that Barr should not give Trump any "sneak preview".

The Mueller drama, filled with unprecedented allegations of collusion or even treason by a US president in league with Moscow, has dogged Trump since he took office following his surprise election defeat of Hillary Clinton.

Throughout, he has maintained that he is the victim of a "witch hunt", while Democratic opponents, who won control of the lower house of Congress last year, say Trump has yet to adequately explain his links to Russia.

Mueller, a Vietnam war veteran and former FBI director, worked in near total secrecy for two years. With his mission as special counsel wrapping up, it is now up to Barr, appointed by Trump, to decide how much of the report to make public.

Public and political pressure for full disclosure is intense and Barr said he is "committed to as much transparency as possible".

There was one key piece of information already confirmed by the justice department, however: Mueller is not recommending any further indictments.

Over the course of his probe, Mueller charged three-dozen individuals and entities, including 25 Russians and six former Trump aides.

But the news that no more indictments are planned means potentially vulnerable figures close to the president, including his son Donald Trump Jr and powerful son-in-law Jared Kushner, will likely rest easier this weekend.

Trump himself made no comment from his Mar-a-Lago golf club resort in Florida, while he awaited the report. His spokesperson Sarah Sanders said the White House now looks "forward to the process taking its course".