The special counsel who investigated Joe Biden's handling of classified documents defended his controversial remarks about the US president's memory on Tuesday, and his decision not to file criminal charges.

Robert Hur's testimony before a congressional committee quickly turned into a partisan slugfest with Democrats and Republicans seizing on the contrasting behaviour of Biden and Donald Trump, who has been indicted for his own mishandling of top-secret documents.

In his nearly 350-page report released in February, Hur said criminal charges against Biden were not warranted and -- in a politically explosive section -- described him as "a sympathetic, well-meaning, elderly man with a poor memory."

Republicans have particularly focused on Hur's comments about Biden's memory, hoping to reignite the age issue for the 81-year-old Democrat ahead of an expected rematch against 77-year-old Republican Trump in the November presidential election.

"My task was to determine whether the president retained or disclosed national defense information 'willfully' -- meaning, knowingly and with the intent to do something the law forbids," Hur told the House Judiciary Committee. "For that reason, I had to consider the president's memory and overall mental state.

"My assessment in the report about the relevance of the president's memory was necessary and accurate and fair," Hur said. "I did not sanitize my explanation. Nor did I disparage the president unfairly."

Democratic lawmakers pushed back against Hur, a 51-year-old registered Republican and career prosecutor, accusing him of making "gratuitous" remarks about Biden's memory and injecting himself into the presidential campaign.

"Despite clearing President Biden from being prosecuted you used your report to trash and smear President Biden," said Representative Hank Johnson, a Democrat from Georgia.

"You cannot tell me you're so naive as to think your words would not have created a political firestorm?" said Adam Schiff, from California.

"You were not born yesterday. You understood exactly what you were doing," Schiff said. "It was a choice. You certainly didn't have to include that language."

Hur, who was appointed special counsel by Biden's attorney general, Merrick Garland, insisted that "partisan politics had no place whatsoever in my work."

"Politics played no part," he said.

'Documents under his bed'

Representative Matt Gaetz and other Republican lawmakers meanwhile contrasted the decision not to charge Biden with the indictment of Trump.

"Biden and Trump should have been treated equally," Gaetz said. "They weren't. And that is the double standard that I think a lot of Americans are concerned about."

Representative Jamie Raskin, a Democrat from Maryland, countered that the circumstances surrounding the mishandling of classified documents by Biden and Trump were not remotely comparable.

"(Biden) did not hide boxes of documents under his bed or in a bathtub," Raskin said.

Trump has been charged in Florida by a different special counsel, Jack Smith, with hoarding classified documents at his Mar-a-Lago home after leaving the White House and obstructing the efforts of the FBI to retrieve them.

Republican and Democratic lawmakers also repeatedly played unflattering video clips during Tuesday's hearing of Biden and Trump making verbal flubs in public appearances and interviews.

Transcripts of Biden's interviews with Hur were released, meanwhile, which appear to give a more balanced picture of their encounter than either the president or the prosecutor gave at the time the report was released.

Whereas Hur said Biden could not remember "even within several years" when his son Beau died of brain cancer, the president in fact remembered the exact date and month, but not the year.

Biden also asked aloud at one point when Trump was elected president and twice about the dates of his vice presidency.

The rest of the transcript often shows a spirited back and forth between the special counsel and the president as they discuss the handling of classified documents while Biden was vice president.

The garrulous Biden frequently joked and digressed into detailed stories, including one about an archery contest during a visit to Mongolia and others about his Corvette car, kept in the garage where some of the documents were found.