Spicer resigned in opposition to Trump's naming of Anthony Scaramucci, a Wall Street financier and longtime supporter of the billionaire investor-turned-president, as the new White House communications director, a White House official told AFP.

Mark Corallo, who coordinated the Trump legal team's public response to the crisis over a probe into possible campaign collusion with Moscow, also stepped down, according to an email he sent to AFP.

Spicer's press briefings -- often combative affairs with White House reporters -- became increasingly infrequent in recent months, with deputy spokeswoman Sarah Huckabee Sanders stepping in to address journalists, often in off-camera briefings.

Sanders however was due to go in front of the cameras later Friday.

Spicer's departure dramatically escalates the tensions within the administration over the direction the investigation is taking, and how the White House is responding.

No reason was given for Corallo's departure, and Spicer so far has remained publicly silent. But the moves come after Trump waded into potentially perilous legal territory by warning investigators not to look into his family finances.

In an expansive interview with The New York Times earlier this week, Trump appeared to make that a red line for special counsel Robert Mueller.

Mueller is examining whether Trump or his aides colluded with Russia's apparent efforts to help tilt the 2016 presidential election in Trump's favor.

Trump has repeatedly denied any wrongdoing, but has struggled to explain why his eldest son and key aides met Russian operatives who promised dirt on Hillary Clinton.

- 'Fundamental line' -

With the investigation apparently extending to financial transactions, US media reported that Trump allies were looking into issuing presidential pardons and for ways to discredit Mueller's investigation.

Trump himself has suggested that Mueller -— a widely respected former FBI director —- may have a conflict of interest.

"There is NO basis to question the integrity of Mueller or those serving with him in the special counsel's office," said former attorney general Eric Holder.

"Trump cannot define or constrain Mueller investigation. If he tries to do so this creates issues of constitutional and criminal dimension."

The White House has pointedly refused to rule out the possibility that Trump would fire Mueller -- an act that would prompt a political firestorm and perhaps a constitutional crisis.

Trump has already fired his FBI director James Comey over the Russia investigation and lashed out at his own attorney general Jeff Sessions for recusing himself from the probe.

Adam Schiff, the leading Democrat on a House of Representatives intelligence committee that is separately investigating Russian actions around the time of the election, also warned that Trump was wandering into dangerous territory.

"There is no doubt that Mueller has the authority to investigate anything that arises from his investigation into the Trump campaign's ties to Russia, including financial links," Schiff said.

The top Democrat on the Senate's intelligence committee, Mark Warner, warned that pardoning anybody who may have been involved "would be crossing a fundamental line."

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