An investigation into Russian interference in the 2016 election moved into a new and more perilous stage for the White House Tuesday after three aides to Donald Trump's presidential campaign, including a former chairman, were charged.

Ex-campaign chairman Paul Manafort and another former Trump aide appeared in court, pleading not guilty to conspiracy against the United States, money laundering and several other charges after the indictments in the Russia probe were unsealed.

The pair was released on bail of $10 million and $5 million respectively and placed under house arrest.

Separately, another former Trump campaign aide, George Papadopoulos, pleaded guilty to lying to the FBI about his Kremlin-related contacts, according to a plea deal revealed Monday.

The unsealed indictments were an explosive opening salvo from independent counsel Robert Mueller, who is leading the Russia probe, after months of speculation, spin and obfuscation about possible Trump campaign collusion with Moscow.

While falling short of providing a smoking gun for top-level conspiracy, the charges point to a potential pattern of senior Trump associates looking to Russia and its proxies for political and economic gain.

Manafort, 68, and Rick Gates, 45, were charged with allegedly hiding millions of dollars gleaned from work with Ukrainian politician Viktor Yanukovych and his pro-Moscow political party.

Papadopoulos, a former Trump foreign policy advisor, admitted he tried to hide contacts with a Moscow-linked professor who was offering “dirt” on Trump's election rival Hillary Clinton.

The revelations prompted a furious and defiant reaction from Trump, who dismissed allegations of collusion and called on Clinton to be investigated.

'No Collusion!'

“Sorry, but this is years ago, before Paul Manafort was part of the Trump campaign. But why aren't Crooked Hillary & the Dems the focus?” Trump tweeted.

"...Also, there is NO COLLUSION!” But the charges signal a dramatic new phase in Mueller's investigation, one that holds grave peril for the Trump presidency.

Papadopoulos revealed that he informed Trump and others personally that he could organize a meeting between the then candidate and Russian President Vladimir Putin.

The ex-advisor told the FBI that he had been instructed by an unnamed “campaign supervisor” to meet Russian officials “off the record” if “feasible.” His contacts with Russian sources came to include Putin's niece and the Russian ambassador in London.

White House spokeswoman Sarah Huckabee Sanders suggested Trump did not recall “specific details of the meeting” and that Papadopoulos had only a limited role.

“It was extremely limited; it was a volunteer position. And again, no activity was ever done in an official capacity on behalf of the campaign in that regard.”

US intelligence agencies have concluded that Putin ordered a vast influence campaign to help Trump win election, including the hack and release of Democratic Party and Clinton campaign emails.

As Mueller's probe has rumbled forward, Trump and sympathetic media organisations like Fox News have increasingly called the former FBI director's independence into question.

Democrats — who dismiss counter-allegations against Mueller and Clinton as a blatant attempt to divert attention — called for the special counsel to be protected.

“The president must not, under any circumstances, interfere with the special counsel's work in any way,” said top Senate Democrat Chuck Schumer.

Internet giants weigh in

Internet giants were expected to tell Congress this week that Russian-backed content aimed at manipulating US politics during last year's election was more extensive than first thought.

Facebook, Google and Twitter were slated to share what they have learned so far from digging into possible connections between Russian entities and posts, ads, and even videos shared on YouTube.

Facebook will tell Congress that some 126 million US users, a potentially large portion of the voting public here, may have seen stories, posts or other content from Russian sources, according to tech news site Recode, the Wall Street Journal and other US media.

The reach is far broader than had originally been estimated by the world's leading social network.