This week could mark a milestone in the long-running investigation into any links between Donald Trump's presidential campaign and Russia.

A grand jury in Washington has approved the first charges in the probe led by independent prosecutor Robert Mueller.

Details of the charges, and who will face them, were not known, but the fact they have been made marked a major step forward in the sweeping investigation. Any person charged could be taken into custody as early as tonight NZT.

How would the President react?

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Twitter was alive with speculation about possible fallout. Judd Legum of Thinkprogress.org tweeted: "Mueller reportedly files first charges in Russia probe. Key Trump allies call on Mueller to resign. Buckle up."

Washington Post columnist Jennifer Rubin tweeted: "R's should be saying publicly that efforts to fire Mueller and/or pardon indicted figures will commence impeachment proceedings." Writer Sarah Kendzior tweeted: "Note the absurd horror of there being SO MANY shady conspirators within Trump's camp that we can't predict with confidence who was charged."

IN THE FRAME

The special counsel's team has conducted interviews with former White House chief of staff Reince Priebus, former spokesman Sean Spicer and other current and former White House officials.

Paul Manafort: In July, FBI agents raided the home of Trump's former campaign manager, whose financial and real estate dealings and prior work for a pro-Russian political party in Ukraine are being investigated by Mueller's team. Last week the Wall Street Journal reported that Manafort was under investigation by the Manhattan US attorney's office for possible money laundering.

Michael Flynn: Flynn was an adviser to Trump's campaign and later briefly served as his national security adviser. Flynn was fired from that post in February after misleading Vice-President Mike Pence about the extent of his conversations with Russian ambassador Sergei Kislyak last year. Flynn also had a US$600,000 contract from a Turkish businessman to help discredit US-based Turkish cleric Fethullah Gulen. NBC News reports that Flynn's son Michael Flynn jnr has also been scrutinised.

Carter Page: Senate investigators quizzed the former Trump foreign policy adviser at the weekend. The FBI at one point had a FISA warrant to monitor his communications.

Michael Cohen: The Trump lawyer testified before the Congress intelligence committees last week.

Jared Kushner: The White House adviser and Trump son-in-law has had to file at least three updates to his national security questionnaire after failing to disclose meetings with Russians.

Donald Trump jnr: The President's son could have violated campaign finance law when he sought help during the campaign from Russians.

- Telegraph Group Ltd, Reuters, AAP