US President Donald Trump on Saturday shrugged off the bombshell news that his former national security adviser pleaded guilty to lying to the FBI and will cooperate with a special prosecutor probing Russian election meddling.

As the Russia probe overshadowed a major legislative win, the Senate's passing of landmark tax cuts, Trump again insisted he and his campaign had not colluded with Russia.

Trump also suggested he has been holding back something important regarding what he knew at the time of ex-security adviser Michael Flynn's firing.

"I had to fire General Flynn because he lied to the Vice President and the FBI. He has pled guilty to those lies. It is a shame because his actions during the transition were lawful. There was nothing to hide!" Trump tweeted.