Flynn's conversations with Sergey Kislyak, the Russian ambassador to U.S. was a primary focus of the Yates's testimony.
Flynn was forced to resign after it was alleged that he discussed the sanctions imposed on Russia by President Obama in a phone conversation with Russian Ambassador Kislyak.
In a highly anticipated hearing, former acting attorney general Sally Yates testified for the first time about why she notified White House officials that President Trump's newly named national security adviser could be compromised.
White House press secretary Sean Spicer on Tuesday defended the administration's decision to wait almost three weeks to dismiss former National Security Adviser Michael Flynn after the Justice Department warned he could be susceptible to Russian blackmail.
His aides are engaging in real-time political combat with Mr Trump, including revealing on Monday that Mr Obama personally warned his successor against hiring embattled Michael Flynn as his national security adviser.
The Senate now in the middle of an investigation into the potential ties between Donald Trump's campaign and Russian operatives, so to add extra emphasis to his claims that there was no collusion, the president temporarily changed his Twitter header image to one of his tweets about the case. "Sally Yates made the fake media extremely unhappy today -- she said nothing but old news!" he said in other tweets. Hours later, NBC News reported that Flynn withheld information from the Defense Intelligence Agency when the Obama administration renewed his security clearance in April 2016.
Yates, who was sacked by President Trump on January 30, said that it is believed that the Russian government had recordings of Flynn's call.
He told reporters Yates was "not exactly a supporter of the president's agenda", an apparent reference to her firing over ordering the Justice Department to not defend Trump's initial executive order banning individuals from seven predominantly Muslim countries from entering the United States.
She said he "essentially could be blackmailed" by the Russians because he apparently had lied to his bosses about his contacts with Moscow's ambassador in Washington.
While Democrats in the Senate Committee and critics of Mr. Trump sought to focus on the delay by the White House in sacking Mr. Flynn, Republicans and the President himself wanted to press on issues related to the leaking of classified information.
Yates filled in new details of the events of January 26, describing contacting McGahn in the morning and telling him she had something sensitive to discuss in person.
And it is of a piece with the person-to-person message President Obama delivered Trump about Flynn in the two's November 10 sit-down - an extraordinary note of caution on a personnel matter for an incumbent President to aim at his successor.
Her testimony goes against with how the White House portrayed her "heads-up" of the potential conflict, as Press Secretary Sean Spicer described it. That's why Republican senators only seem to care about the sources of those media reports about the Trump national security adviser being compromised by the Russians rather than the fact that the Trump national security adviser was compromised by the Russians.
She said she was briefing the Trump White House so that officials could take "the action that they deemed appropriate" and that she believed the Russians already had the same information. Yates testified alongside James Clapper, a top intelligence chief during the Obama administration.