Speaking from the South Lawn, Trump issued a flat denial: "I never worked for Russia." He blasted former FBI and Justice Department officials and repeated his claim that the investigation into his ties to Moscow is a hoax.
Trump raised eyebrows over the weekend when he didn't directly answer the Russia question in an interview with Fox News host Jeanine Pirro, a personal friend.
He was asked about a New York Times report that law enforcement officials began investigating, in 2017, whether Trump had been working on behalf of Russia against U.S. interests.
Trump said the question was "insulting," but did not directly deny it.
Monday's clear denial demonstrated the unprecedented pressure Trump faces as the special counsel's Russia investigation shows possible signs of wrapping up. After two years in office, the president is still being questioned about whether he was compromised by Russian intelligence agencies and he is still struggling to answer clearly.
White House aides expressed regret over the weekend that the president did not more clearly and forcefully deny being a Russian agent on Saturday when asked by the usually friendly Fox News host, according to three White House aides and Republicans close to the White House. The three spoke on condition of anonymity because they were not authorized to discuss private conversations.
When Pirro asked if he had ever worked for Russia, Trump said it was "the most insulting thing I've ever been asked."
"I think it's the most insulting article I've ever had written, and if you read the article you'll see that they found absolutely nothing," he said.
Trump went on to assert that no president has taken a harder stance against Russia than he has.
"If you ask the folks in Russia, I've been tougher on Russia than anybody else, any other ... probably any other president, period, but certainly the last three or four presidents," he said.
On Monday, Trump also defended his decision to fire FBI Director James Comey, a move that has drawn Mueller's scrutiny. Trump called the Russia probe "a whole big fat hoax."
Special counsel Robert Mueller is looking into Russian election interference and whether Trump's campaign coordinated with the Russians. He also is investigating possible obstruction of justice by Trump.