Image copyright Reuters Image caption Attorney General Barr wrote a summary of the Russia report before the full version was released

US Special Counsel Robert Mueller wrote to the country's top lawyer to express frustration at his summary of the Russia investigation.

He told Attorney General William Barr in March that his four-page summary lacked "context".

Mr Barr's summary said there was no collusion between Russia and the Trump campaign, but it did not exonerate the president of obstruction of justice.

Mr Mueller has agreed to testify in Congress later in May.

According to Mr Mueller's letter released on Wednesday, he has twice requested that Mr Barr reveal more information about his investigation's conclusions.

He said the additional information was necessary as Mr Barr's original summary led to "public confusion about critical aspects of the results of our investigation".

A justice department spokeswoman said in a statement that "the Special Counsel emphasised that nothing in the Attorney General's [summary] was inaccurate or misleading".

"But he expressed frustration over the lack of context... regarding the [report's] obstruction analysis," she said.

Mr Mueller's investigation into alleged Russian interference in the 2016 presidential election has led to 35 people being charged, including several who were a part of Mr Trump's campaign and administration.

Mr Barr, who was appointed by Mr Trump, held a news conference before the full report was made public in which he backed the president.

But the mammoth document was released on 18 April in redacted form and senior Democrats said the attorney general's summary had been "misleading".

What did Mueller say?

In his letter to Mr Barr, Mr Mueller said his summary "did not fully capture the context, nature and substance of this office's work and conclusions".

"There is now public confusion about critical aspects of the results of our investigation," he said.

"This threatens to undermine a central purpose for which the Department appointed the Special Counsel: to assure full public confidence in the outcome of the investigations," he added.

In a statement, the Justice Department said Mr Mueller had been frustrated over a lack of context in media coverage.

The news of Mr Mueller's letter came shortly before Mr Barr appeared before Congress.

Media playback is unsupported on your device Media caption The Mueller report - in 60 seconds

He is appearing before the Senate Judiciary Committee on Wednesday and the equivalent House committee on Thursday.

Democrats are expected to question him on his handling of the Russia report.

Moments before the hearing began, House Judiciary Committee Chairman Jerry Nadler told reporters that Mr Mueller and the committee had reached an agreement for him testify later in May, although no specific date has yet been chosen.

A number of top Democrats, including House Speaker Nancy Pelosi, have strongly criticised Mr Barr in recent days.

"Attorney General Barr misled the public and owes the American people answers," Ms Pelosi wrote on Twitter.