US PRESIDENT Donald Trump has said the actions of former national security adviser Michael Flynn after the 2016 presidential election were lawful.

Flynn, who has pleaded guilty to making false statements to the FBI about contacts with Russia's then ambassador Sergei Kislyak in December 2016, has entered into a plea deal, agreeing to co-operate with an inquiry into alleged collusion between Trump's team and Russian officials.

Earlier the president tweeted that he fired Flynn "because he lied to the Vice President and the FBI" and insisted he had "nothing to hide".

Under the terms of the deal Flynn pleaded to a lesser charge and a shorter sentence than he might have otherwise faced, prompting speculation that he has incriminating evidence that could damage senior members of the Trump team, including Trump's adviser and son-in-law Jared Kushner.