Senior Democratic senator Elizabeth Warren has warned that the allegations around the Trump administration and its ties with Russia could be worse than the Watergate scandal that resulted in the resignation of Richard Nixon.

Warren (D-Mass), was speaking in San Francisco at a liberal advocacy group when she discussed the current situation at the White House.

She said: "A lot of people compare this to Nixon and they say 'That's what got Nixon, was the obstruction of justice.' And it's true, but this is a lot worse because this one has connections to a foreign power, which the whole back-and-forth around obstruction of justice with Nixon did not include.

"First, we make sure that we've gotten the facts, that you get them out there, but ... we've got to turn the heat up under Republicans, guys, you don't get to take a pass on this, you've got to step up on this one."

Since the sacking of the FBI director James Comey, the scandal surrounding Trump, the White House, the 2016 campaign and Russia has escalated on a nearly daily basis.

Comey is expected to testify publicly in the US senate next week, where he is expected to confirm the accusations that Trump pressured him to end the investigation into possible collusion between Trump's aides and Russia.

Specific details about the testimony are yet to be released but it is expected that Comey will appear before the Senate Intelligence Committee who are currently investigating possible connections between the Trump campaign and Russia during last year's presidential election.

Senior Republican senator John McCain said in May that the size and scope of the scandal was reaching the same size as Watergate.