House Speaker Nancy Pelosi on Thursday accused Attorney General William Barr of committing a crime by lying to Congress about Robert Mueller's report and Mueller's issues with how Barr has characterized the special counsel's findings.

"What is deadly serious about it is the attorney general of the United States of America is not telling the truth to the Congress of the United States. That's a crime," the California Democrat told reporters.

Pressed again about the accusation, Pelosi said, "He lied to Congress. If anybody else did that, it would be considered a crime. Nobody is above the law." Asked whether Barr should go to jail, the speaker responded that "there's a process involved here."

In a statement in response, Justice Department spokeswoman Kerri Kupec said Pelosi's "baseless attack on the Attorney General is reckless, irresponsible and false."

Pelosi's comments appeared to reference answers Barr gave during House testimony last month. Lawmakers asked him about reported frustrations Mueller's team had with a summary the attorney general wrote about the special counsel's report.

Barr said he was not aware of concerns the Mueller team had about his summary. But news reports revealed this week that Mueller had written a letter to Barr expressing concerns about how the attorney general depicted the "substance" of the report — before the attorney general testified.

Here is the exchange from an April 9 hearing that apparently sparked Pelosi's accusation.

Rep. Charlie Crist, D-Fla.: "Reports have emerged recently, general, that members of the special counsel's team are frustrated at some level with the limited information included in your March 24 letter. ... Do you know what they are referencing with that?" Barr: "No, I don't. I think I think, I suspect that they probably wanted more put out, but in my view I was not interested in putting out summaries."

The comment from the highest-ranking Democrat in the country intensifies the party's criticism of the top U.S. law enforcement official. While Pelosi and Senate Minority Leader Chuck Schumer have accused Barr of protecting Trump and having a conflict of interest, neither lawmaker has previously gone as far as alleging a crime.

Pelosi has so far resisted calls to impeach the president, which have intensified following the release of Mueller's redacted report. Schumer, meanwhile, is writing a letter to Barr questioning the attorney general's views on executive power, particularly his suggestion that a president could end an investigation if they feel they are falsely accused, NBC News reported. The senator wrote that "if these views are truly your views, you do not deserve to be Attorney General."

Democrats want Mueller to testify publicly and have questioned why the Justice Department decided not to charge the president with obstructing justice by trying to influence Mueller's investigation. While Mueller's report declined to say whether Trump obstructed justice, he also noted that the report did not "exonerate" Trump. It also set out a detailed case for Congress to potentially investigate the president for obstruction.

Pelosi's remarks Thursday follow a House Judiciary Committee hearing that Barr decided not to attend. The panel's chairman, Democratic Rep. Jerry Nadler of New York, threatened to hold the attorney general in contempt of Congress if he does not give access to Mueller's full, unredacted report on the Russia probe.

Barr repeatedly defended Trump during a Senate Judiciary Committee hearing on Wednesday. The president has contended Mueller's investigation fully exonerates him on both questions of whether he obstructed justice and whether his campaign coordinated with Russian efforts to influence the 2016 election. Trump repeatedly described the investigation as a "witch hunt" and has since falsely called the probe a "coup."

This week's events have escalated a partisan battle over congressional witnesses who have testified about the investigation and the president's conduct. Democrats shot down a GOP resolution Wednesday to refer Trump's former lawyer and fixer Michael Cohen to the Justice Department for accusations of lying to Congress in February.

— CNBC's Kevin Breuninger and Dan Mangan contributed to this report

Subscribe to CNBC on YouTube.