U.S. President Donald Trump on Thursday ordered the intelligence community to co-operate with Attorney General William Barr's review of the events that prompted an investigation into links between the Trump campaign and Russia.

The directive comes as the White House spars with congressional Democrats over the work of special counsel Robert Mueller, who led a two-year investigation into whether Russia meddled in the 2016 U.S. election and if there were any ties between the Trump campaign and Russia.

"Today, at the request and recommendation of the attorney general of the United States, President Donald Trump directed the intelligence community to quickly and fully co-operate with the attorney general's investigation into surveillance activities during the 2016 presidential election," White House spokesperson Sarah Huckabee Sanders said after Trump issued the directive.

The order also allows Barr to declassify any information he sees fit during his review.

A redacted version of Mueller's report was released publicly in April. The report stated that the probe could not establish that members of the Trump campaign engaged in a criminal conspiracy with Russia, though the campaign "expected it would benefit electorally from information stolen and released through Russian efforts." The report did not draw a conclusion on whether Trump obstructed justice, but outlined some incidents that Democrats have said may constitute obstruction.

Several Trump associates were found to have lied to investigators, with prison sentences for various offences resulting for his campaign chair Paul Manafort, personal lawyer Michael Cohen and foreign policy adviser George Papadopoulos. The Mueller report also stated some of those who were investigated and associated with the Trump campaign "deleted relevant communications," hindering some investigative avenues.

Barr released a four-page summary of the Mueller report that was widely criticized, but speaking to reporters before leaving on a trip to Japan, Trump called the attorney general "one of the most respected people in this country."

He's going to look at a lot of documents — some he might find interesting, maybe he'll find none interesting. - Donald Trump on William Barr's mandate

"He's going to look at a lot of documents — some he might find interesting, maybe he'll find none interesting," said Trump.

Republican House member Mark Meadows of North Carolina tweeted on Thursday that "Americans are going to learn the truth about what occurred at their Justice Department."

Adam Schiff, a Democrat member and chairman of the House's intelligence committee, criticized Trump's directive.

"While Trump stonewalls the public from learning the truth about his obstruction of justice, Trump and Barr conspire to weaponize law enforcement and classified information against their political enemies," Schiff said on Twitter.

"The coverup has entered a new and dangerous phase. This is un-American," he added.

Democratic Congressman Don Beyer of Virginia concurred. Beyer said giving Barr the ability to declassify any information after the newly installed attorney general edited the Mueller report for public consumption was "deeply corrupt."

The Inspector General has been investigating this for months — but he’s independent, not a Trump hack willing to subvert the Justice Department. That’s why Trump wants Barr on it.<br><br>Trump and Barr want this declassified while hiding Mueller report redactions — it’s deeply corrupt. <a href="https://t.co/lhyzjjNaV6">https://t.co/lhyzjjNaV6</a> —@RepDonBeyer

Very excited Americans will now receive a ton more info about the origins of the Trump-Russia collusion probe. Let’s see the scope memo, Page FISA Apps, Ohr 302s & more. Never again should the justice system be weaponized like this or Americans’ intelligence be insulted like this —@RepLeeZeldin

But New York Congressman Lee Zeldin championed the new look, stating, "never again should the justice system be weaponized like this or Americans' intelligence be insulted like this."

In separate comments late Thursday on MSNBC's The Rachel Maddow Show, House judiciary committee chair Jerry Jerrold Nadler said Mueller had told him he is willing to testify to lawmakers in private, but is willing to make a public statement.

Nadler, a Democrat, told Maddow that if Mueller did testify behind closed doors, the public would get a written transcript of the testimony.

But Trump again railed on Twitter early Friday against the Mueller investigation, and said the investigation was politically motivated.

Trump told reporters the Democrats were trying for a "do-over" of an investigation that didn't provide the outcome they wanted.

"They have to get over their anger," said Trump.

"Why does [Mueller] have to testify? It's ridiculous."

Wray, Trump differ on 'spying'

Trump, a Republican, harbours suspicions the Democratic Obama administration ordered him investigated during the 2016 campaign to try to undermine his candidacy, and he wants payback against those he believes were responsible.

"Comey, Brennan, Clapper, we're draining the swamp, folks," Trump told a rally on Monday in Pennsylvania, referring to former FBI director James Comey, former CIA director John Brennan and James Clapper, a former director of national intelligence, all of whom have been critical of Trump.

FBI head Christopher Wray was nominated by Donald Trump after the dismissal of James Comey, but he earned criticism from the president after recent testimony he gave on Capitol Hill. (REUTERS)

Of specific interest to Trump are the warrants that emanated from a secretive court that authorizes surveillance on foreign powers and their agents. Trump supporters believe the warrants will identify those responsible for the Russia probe that is still roiling Washington.

Last month, Barr said at a Senate hearing that "spying" on Trump's campaign was carried out by U.S. intelligence agencies, though he later referred to his concerns as focused on "unauthorized surveillance."

Barr has assigned a top federal prosecutor in Connecticut to probe the origins of the Russia investigation in what is the third known inquiry into the opening of the FBI probe.

Watch: Barr held in contempt (May 12)

U.S. lawmakers pushed through a measure to approve placing William Barr in contempt of Congress for refusing to hand over the full Mueller report. Plus, Donald Trump's personal lawyer Rudy Giuliani cancels a planned trip to Ukraine. Linda Feldmann with the Christian Science Monitor, and Jill Colvin from The Associated Press, speak with Michael Serapio on CBC News Network. 11:51

In separate testimony recently, FBI Director Christopher Wray said in a response to a lawmaker's question that he believed court-approved surveillance does not amount to spying.

Wray's opinion earned a rebuke from the president a few days later.

Investigate whatever you wish about 2016 but don’t forget the people of the FBI must investigate and stop Russian efforts in the 2020 election. What impact will loose talk about “spying” and disgraceful talk about “treason” have on FBI agents and analysts? —@Comey

Trump said he "thought it was a ridiculous answer."

James Comey, the FBI director Trump fired in May 2017, said on social media he was worried about the effect of Trump's rhetoric on the FBI and on efforts to fight foreign interference in next year's presidential election.