WASHINGTON: US President Donald Trump on Friday lashed out at the FBI inquiry into possible collusion between Moscow and his campaign, and refused to rule out pardoning former aide Michael Flynn, who pleaded guilty to lying about his contacts with Russia.

Trump said it was too early to discuss any pardon for his ex-national security advisor - which would prompt a political firestorm.

So far, the FBI's sprawling probe into Russia's meddling in the 2016 election has ensnared at least four members of his campaign inner circle.

"I don't want to talk about pardons for Michael Flynn yet," Trump said before leaving the White House to give remarks at a graduation ceremony at the Federal Bureau of Investigation´s academy in Quantico, Virginia.

"I can say this - when you look at what's gone on with the FBI and with the Justice Department, people are very, very angry," he said.

"When everybody - not me, when everybody - the level of anger at what they´ve been witnessing with respect to the FBI is certainly very sad."

US intelligence agencies have concluded that Russian President Vladimir Putin ordered his intelligence and cyber services to help turn the election in Trump´s favor.

Amid mounting evidence of campaign contacts with Russian operatives, Trump repeated his denial Friday.

"There is absolutely no collusion. I didn´t make a phone call to Russia. I have nothing to do with Russia. Everybody knows it. That was a Democrat hoax. It was an excuse for losing the election," he said.

Trump also blasted as "disgraceful" recently released text messages exchanged by an FBI agent and a lawyer involved in the probe who were critical of the president.

The agent, Peter Strzok, has been removed from the investigation being led by special counsel Robert Mueller, a former FBI director.