The speaker of the U.S. House of Representatives, Republican Paul Ryan, said Tuesday that he has received assurances that the special counsel for the Russia investigation, Robert Mueller, will not be fired, following growing rumors that President Donald Trump would like to do just that.

"I have received assurances that his firing is not even under consideration. We have a system based upon the rule of law in this country," Ryan told a press conference.

Though the White House insists that firing Mueller is not an option, the latest statements and indications from President Trump have increased concern in Washington that he could terminate Mueller at any time

The president stepped up his attacks on the investigation into Russia's possible interference in the last presidential election, from which he emerged the winner, and this weekend on Twitter he specifically slammed Mueller by name for the first time.

Over the last few days, the billionaire has expanded his legal team, and Attorney General Jeff Sessions dismissed former FBI Director Andrew McCabe a scant two days before his retirement, after numerous calls from Trump on the matter.

Many Republicans see getting rid of Mueller as a big political risk, but without introducing any legislation to shield him from a presidential decision to do so.

Ryan, however, has insisted that Mueller should be able to complete his investigation and expressed confidence that he will.

"The special counsel should be free to follow through with his investigation to its completion without interference. Absolutely, I am confident that he will be allowed to do that," said Ryan, who, unlike other leaders of his party, rejected the notion of naming a second special counsel.

Mueller's credentials stand in his favor after years at the head of the FBI, not to mention the fact that his ideological line is more in tune with Republicans, and that in 2001 he was named director of the Federal Bureau of Investigation by a president of that party, George W. Bush.