FBI Director James Comey confirmed Monday that the bureau is investigating possible links and co-ordination between Russia and associates of U.S. President Donald Trump as part of a broader probe of Russian interference in last year's presidential election.

The extraordinary revelation, and the first public confirmation of an investigation that began last summer, came at the outset of Comey's opening statement in a congressional hearing examining Russian meddling and possible connections between Moscow and Trump's campaign.

He acknowledged that the FBI does not ordinarily discuss continuing investigations, but said he'd been authorized to do so given the extreme public interest in this case.

"This work is very complex, and there is no way for me to give you a timetable for when it will be done," Comey told the House intelligence committee.

The hearing, providing the most extensive public accounting of a matter that has dogged the Trump administration for its first two months, quickly broke along partisan lines. Democrats pressed for details on the status of the FBI's investigation, while Republicans repeatedly focused on news coverage and possible improper disclosures of classified information developed through surveillance.

'No information that supports those tweets'

Under questioning from the committee's top Democrat, Representative Adam Schiff, he also publicly contradicted a series of tweets from Trump that declared the Republican candidate's phones had been ordered tapped by President Barack Obama during the campaign.

"I have no information that supports those tweets, and we have looked carefully inside the FBI," Comey said. The same was true, he added, of the Justice Department.

He also took issue with Trump tweets sent out during the hearing, including one that said, "The NSA and FBI tell Congress that Russia did not influence electoral process."

The NSA and FBI tell Congress that Russia did not influence electoral process. <a href="https://t.co/d9HqkxYBt5">pic.twitter.com/d9HqkxYBt5</a> —@POTUS

The president posted seven tweets during Comey's questioning from the @POTUS account.

Comey also disputed allegations that British intelligence services were involved in the wiretapping.

Comey was the latest government official to reject Trump's claims, made without any evidence, that Obama had wiretapped his New York skyscraper during the campaign. Representative Devin Nunes, a California Republican and chairman of the House intelligence committee, also rejected it earlier in the hearing.

Comey was testifying along with National Security Agency Director Michael Rogers.

Trump sends out pre-emptive tweets

Trump took to Twitter before the hearing began, accusing Democrats of making up allegations about his campaign associates' contact with Russia during the election. He said Congress and the FBI should be going after media leaks and maybe even Hillary Clinton instead.

The Democrats made up and pushed the Russian story as an excuse for running a terrible campaign. Big advantage in Electoral College & lost! —@realDonaldTrump

"The real story that Congress, the FBI and others should be looking into is the leaking of classified information. Must find leaker now!" Trump tweeted early Monday as news coverage on the Russia allegations dominated the morning's cable news.

Trump also suggested, without evidence, that Clinton's campaign was in contact with Russia and had possibly thwarted a federal investigation. U.S. intelligence officials have not publicly raised the possibility of contacts between the Clintons and Moscow. Officials investigating the matter have said they believe Moscow had hacked into Democrats' computers in a bid to help Trump's election bid.

Distance from former advisers

As Comey's hearing continued, the White House distanced itself from two former senior members of Trump's team.

White House press secretary Sean Spicer on Monday referred to Trump's former national security adviser, Michael Flynn, as a "volunteer of the campaign." And he said Paul Manafort, who ran Trump's campaign leading up to the Republican National Convention, "played a very limited role for a very limited amount of time."

In this July 17, 2016 file photo, then-Trump campaign chairman Paul Manafort talks to reporters. Manafort, and the White House, downplayed his role in the campaign and his contacts with Russians. (Matt Rourke/The Associated Press)

Flynn resigned from the White House last month after he was found to have misled senior members of the administration about his contacts with Russia's top diplomat to the U.S.

Manafort resigned from Trump's campaign last summer following allegations of contacts with Russian intelligence officials.

Monday evening, Manafort released a statement defending himself.

In the statement, he said he had "no role or involvement" in the hack of the Democratic National Committee and disclosure of stolen emails. He said he has never spoken to any Russian officials or others who claimed to be involved in the attack.

Manafort said there is "constant scrutiny and innuendo" but "no facts" backing up allegations. He said he's disappointed anyone would legitimize attempts to discredit him and Trump's election.

Public accounting

Monday's hearing, one of several by congressional panels probing allegations of Russian meddling, could allow for the greatest public accounting to date of investigations that have shadowed the Trump administration in its first two months.

National Security Agency Director Michael Rogers, accompanied by FBI Director James Comey, testifies on Capitol Hill in Washington Monday before the House intelligence committee hearing on allegations of Russian interference in the 2016 U.S. presidential election. (Manuel Balce Ceneta/Associated Press)

The top two lawmakers on the committee said Sunday that documents the Justice Department and FBI delivered late last week offered no evidence that the Obama administration had wiretapped Trump Tower, the president's New York City headquarters.

But the panel's ranking Democrat said the material offered circumstantial evidence that American citizens colluded with Russians in Moscow's efforts to interfere in the presidential election.

"There was circumstantial evidence of collusion; there is direct evidence, I think, of deception," Schiff said on NBC's Meet the Press. "There's certainly enough for us to conduct an investigation."

Nunes said, "For the first time the American people, and all the political parties now, are paying attention to the threat that Russia poses."

"We know that the Russians were trying to get involved in our campaign, like they have for many decades. They're also trying to get involved in campaigns around the globe and over in Europe," he said on Fox News Sunday.

The Senate intelligence committee has scheduled a similar hearing for later in the month.