US President Donald Trump insisted there is no evidence he colluded with Russia.

Washington: US President Donald Trump insisted there is no evidence he colluded with Russia after a Senate hearing that highlighted warnings that his former national security advisor was vulnerable to Russian blackmail.

With the issue of the president's ties to Moscow back in the spotlight, Trump took to Twitter to dismiss as "old news" the Senate testimony on Monday by former acting attorney general Sally Yates about his former national security advisor Michael Flynn.

Instead, Trump chose to play up former director of national intelligence James Clapper's acknowledgement during the same hearing that he was not aware of any evidence of collusion between the president and Russia, which American intelligence has concluded tried to sway the US election in Trump's favor.

"Director Clapper reiterated what everybody, including the fake media already knows — there is 'no evidence' of collusion w/ Russia and Trump," Trump said.

Clapper, however, had added that he had not been aware that the issue was under investigation by the FBI until it was publicly revealed in March, suggesting the agency might have evidence he wasn't privy to.

Asked about it, Yates said answering the question would require revealing classified information. But she noted that "you should not draw from that an assumption that that means that the answer is yes."

Yates, a Barack Obama appointee sacked by Trump early in his presidency, took the stand alongside Clapper during the hotly-anticipated three-hour hearing of the Senate Judiciary Committee.

Yates confirmed reports that she had told the White House, six days into Trump's administration, that Flynn, a former military intelligence chief, had not been honest with Vice President Mike Pence about his discussions with the Russian ambassador to Washington, leaving him vulnerable to leverage from Moscow.

It nevertheless took 18 days before the president, pressed by Pence and others, dismissed the retired army lieutenant general, who had advised him on security issues throughout the 2016 presidential campaign.

"We believed that General Flynn was compromised with respect to the Russians," Yates told the hearing in her first public comments on the scandal which has dogged the opening months of Trump's presidency.

"This was a problem because not only did we believe that the Russians knew this but that they likely had proof of this information. And that created a compromise situation, a situation where the national security advisor essentially could be blackmailed by the Russians."

Yates, who was fired on 30 January after defying Trump over his contested travel ban, did not say what Flynn discussed with ambassador Sergey Kislyak in a number of December 2016 phone calls, which were secretly monitored by US intelligence.

Pence said in January that Flynn denied those calls involved sanctions placed on Russia by the Obama administration in response to its election meddling.