Washington: The top Democrat on the Senate Intelligence Committee has warned US President Donald Trump against firing special counsel Robert Mueller or pardoning any targets of the federal probe into Russia and the president's Republican campaign.

In a speech from the Senate floor, Virginia Senator Mark Warner said that growing criticism of Mueller's probe pointed to an effort to lay the groundwork for Mueller's removal or other interference in the federal investigation. Warner said any attempt to remove Mueller would provoke a "constitutional crisis".

Asked this past weekend if he was considering trying to push out Mueller, Trump replied, "No, I'm not." And a White House lawyer has repeatedly said the president is not considering any pardons. Trump cannot directly fire Mueller, but critics say he could demand that top Justice Department officials fire Mueller and then remove them if they don't follow through.

"The president has said he's not considering removing special counsel Mueller, but the president's track record on this front is a source of concern," Warner said. "I'm certain that most of my colleagues believed that he wouldn't fire (former FBI Director) Jim Comey."