White House alerted about Flynn's blackmail risk: Ex

Meanwhile, Justice Department officials have scrutinized the business dealings of Paul Manafort, who resigned in August as Trump's campaign chairman.

Yates and Clapper both swore under oath that they had never leaked classified information.

Flynn, a retired general once seen as a potential Trump vice president, has emerged as a central figure in the Russian probes.

The simmering controversy over Russia's alleged meddling in last year's United States election returns to the spotlight Monday, with a former top Justice Department official set to testify on links between President Donald Trump's advisers and Moscow.

Mr Flynn, a retired army lieutenant-general, misled the Trump administration about discussing USA sanctions against Russian Federation with the country's envoy, Sergei Kislyak, before the inauguration.

Ms Yates said she could not disclose whether her knowledge derived from intercepted U.S. intelligence communications between Mr Flynn and the envoy. Flynn's attorney, the Justice Department, the Federal Bureau of Investigation and the US Attorney's office in Alexandria have all declined to comment on this revelation, which is fine, because honestly, what could they really say?

Former President Barack Obama is starting to define his new role in the age of Donald Trump.

That is because she was sacked on January 30 after refusing to defend the president's executive order banning refugees and travel from several predominantly Muslim countries.

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.

White House press secretary Sean Spicer vehemently denied the reports at the time and said that the White House actively supports Yates' testifying in public. Obama had warned Trump, then president-elect, not to give the post of national security adviser in his administration to Flynn just after the Republican's surprise victory in the November 8 election, a former Obama aide said.

"I will be asking the question that is really on the minds of Americans", Democratic committee member Senator Richard Blumenthal said Monday ahead of the hearing.

"You specifically asked me if the president asked me to do something that was unlawful and unconstitutional, would I say no?" she said.

Congressional committees began investigating after USA intelligence agencies concluded that Russian President Vladimir Putin ordered hacking of Democratic political groups to try to sway the election toward Trump.

He recalled the Democrats who were swept into office with him in 2009, but lost their seats after casting votes in favour of the "Obamacare" measure.

But Mr Spicer said that should not come as a surprise "given that General Flynn had worked for President Obama [and] was an outspoken critic of President Obama's shortcomings specifically as it related to his lack of strategy confronting Isis and other threats around that were facing America". Lindsey Graham's remarks that he wants to look into whether Trump has any business dealings with Russian Federation.

"The onus is on the next generation to lead the party in the Trump era", said Dan Pfeiffer, a longtime Obama White House and campaign adviser.

The revelations did not in themselves represent a development that could break open the mystery of Moscow's meddling with last year's election and increasingly vocal Democratic claims of collusion between Russian Federation and members of the President's inner circle.