WASHINGTON - Congressional Democrats stepped up pressure on President Donald Trump's administration on Thursday as US House of Representatives Speaker Nancy Pelosi accused Attorney General William Barr of committing a crime and an influential committee chairman threatened to hold Barr in contempt of Congress.

In a defiant letter released on Thursday, the White House said Trump has the right to instruct advisers not to testify before congressional oversight probes related to the Special Counsel Robert Mueller's Russia inquiry and called Mueller's report on the matter defective.

The statements mark a sharp escalation in the conflict between Trump's administration and Democrats in Congress, who are weighing whether they should try to remove Trump from office using the impeachment process as they demand information on his taxes, his businesses and other topics.

Democrats accused the Republican president and his administration of a launching a growing attack on American democracy.

Pelosi, the top Democrat in Congress, accused Barr of lying to lawmakers about his interactions with Mueller after the special counsel concluded his 22-month investigation into Russia's efforts to interfere in the 2016 US election to boost Trump's candidacy.

"That's a crime," the top Democrat in Congress told reporters, referring to Barr's congressional testimony.

Justice Department spokeswoman Kerri Kupec called Pelosi's allegation "reckless, irresponsible and false."

The exchange came shortly after House Judiciary Committee Chairman Jerrold Nadler threatened to hold Barr in contempt of Congress if he does not provide a full, unredacted copy of Mueller's findings. That could potentially lead to fines or imprisonment for the top US law enforcement official.

Barr released Mueller's report on April 18, with some parts blacked out to protect sensitive information.

Pelosi and other leaders in the House, which is controlled by Democrats, previously had cautioned against launching the impeachment process, which would begin in the House but would face long odds of success in the Republican-controlled Senate and could alienate voters ahead of the 2020 presidential election in which Trump is seeking re-election.

But Pelosi said the Trump administration was continuing to ignore congressional subpoenas and noted that Congress launched impeachment proceedings against President Richard Nixon in the 1970s after he resisted similar demands. Nixon resigned in 1974.

"I think that the statements being made by the president of the United States as giving a blanket statement that he's not going to honor any subpoenas is obstruction of justice," Pelosi said.

White House legal counsel Emmet Flood, in an April 19 letter to Barr obtained by Reuters on Thursday, said Trump's decision to let advisers cooperate with the Mueller probe does not extend to congressional oversight investigations.

"It is one thing for a president to encourage complete cooperation and transparency in a criminal investigation conducted largely within the Executive Branch. It is something else entirely to allow his advisers to appear before Congress," the letter said.

Democrats have accused Barr of misleading Congress by testifying in April that he did not know whether Mueller agreed with his initial characterization of the report - failing to mention a March 27 letter from the special counsel complaining that Barr's initial public summary did not "fully capture the context, nature and substance of this Office's work." Several Democrats have called on him to resign.

Nadler had set a Wednesday deadline for Barr to hand over the unredacted report and its underlying evidence. Barr canceled his testimony after clashing with Nadler over the hearing's format. Nadler said he will move forward with a contempt citation as soon as Monday.

"The very system of government of the United States - the system of limited power, the system of not having a president as a dictator - is very much at stake," Nadler told reporters after a 15-minute committee session held in place of Barr's appearance.

Democrats have said they may issue a subpoena to try to force Barr to testify before their committee.

"The failure of Attorney General Barr to come to the hearing today is simply another step in the administration's growing attack on American democracy and its attack on the right of Congress" as a co-equal branch of government, Nadler said.

Barr spent four hours before a Republican-led Senate committee on Wednesday defending his handling of Mueller's report on Russia's interference in the election to boost Trump's candidacy and whether the president subsequently tried to obstruct Mueller's probe.

Representative Doug Collins, the House Judiciary Committee's top Republican, said Barr did not appear before that panel because Nadler had insisted on an aggressive format with an extra hour of questioning from its own lawyers, in addition to those from the committee's lawmakers.

"They want it to look like an impeachment hearing because they won't bring impeachment proceedings," Collins said.

The report detailed extensive contacts between Trump's 2016 presidential campaign and Moscow and the campaign's expectation that it would benefit from Russia's actions, which included hacking and propaganda to boost Trump and harm Democratic candidate Hillary Clinton. The report also detailed a series of actions Trump took to try to impede the investigation.

Mueller, a former FBI director, concluded there was insufficient evidence to show a criminal conspiracy and opted not to make a conclusion on whether Trump committed obstruction of justice, but pointedly did not exonerate him. Barr has said he and Rod Rosenstein, the Justice Department's No. 2 official, then determined there was not enough evidence to charge Trump with obstruction.

Reuters