Watch live: Sally Yates testifies on Russia and Flynn

The simmering controversy over Russia's alleged meddling in last year's US 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.

Former acting attorney general Sally Yates, a Barack Obama appointee held over for the start of the new administration, could offer new details about how Trump handled concerns that national security adviser Michael Flynn lied about his ties with Russian officials.

She is expected to tell the Senate Judiciary Committee that she explicitly warned Trump's team in January that Flynn could be compromised by his communications with Russia's US ambassador.

Watch the proceedings live here, beginning at 2:30 p.m. EST:

Yates will be speaking publicly for the first time on what she knew of Russia's interference in the November election, a scandal that has dogged the Republican president from the start of his four-year term.

Trump, who fired Yates soon after taking office, shot off a series of pre-emptive strikes ahead of her appearance on Capitol Hill, accusing Obama of enabling Flynn, and assailing Yates for allegedly leaking classified information.

"General Flynn was given the highest security clearance by the Obama Administration," Trump tweeted early Monday. 

"Ask Sally Yates, under oath, if she knows how classified information got into the newspapers soon after she explained it to W.H. Counsel," he added.

Flynn at center of storm

But just hours before the hearing was to open, Trump's early support for Flynn was challenged by reports that then-president Barack Obama had strongly advised against giving him such a senior White House job.

Obama, who had fired Flynn as defense intelligence chief two years earlier for mismanagement, warned Trump about Flynn's conduct during a first meeting in the Oval Office on Nov.r 10, two days after the election.

The warning was "based on Flynn’s role as head of DIA," a former administration official told AFP.

Weeks later, Yates reportedly warned the White House that Flynn's phone calls with Moscow envoy Sergey Kislyak, which US intelligence secretly monitored, could leave him open to blackmail.

Yet despite those warnings, Trump named the former military intelligence chief his White House national security adviser, only to end up firing him after a month because Flynn had not been forthcoming about those calls.

Yates, known as a tough and independent prosecutor, has been a target of Trump's ire since she refused in January to support his controversial immigration ban on nationals from several Muslim-majority nations. He then fired her.

Also testifying Monday will be former director of national intelligence James Clapper, who has repeatedly warned about the need to get to the bottom of Russia's intervention in the election last year. US intelligence believes Russia sought to damage Trump's rival Hillary Clinton.

"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.

"They want the truth uncovered about Russian interference and the last election and potential collusion of Trump associates," Blumenthal told CNN.

Slow progress in Russia probe

The focus on Russia's alleged "active measures" campaign against Western democracies has intensified following the leak of potentially damaging files from the campaign of Emmanuel Macron just days before he won France's presidential election on Sunday.

While it appeared to have little effect on the French election's outcome, it paralleled the leak of Democratic party files last year ahead of the US vote, which hurt Clinton and boosted Trump.

Trump has repeatedly branded the issue "fake news" despite Clapper and other US intelligence community leaders having concluded that Russian President Vladimir Putin himself was behind the interference operation.

The Senate Judiciary Committee is one of several bodies in Congress probing Russian interference.

But progress has been slow, with Democrats accusing Republicans of stalling the investigation to protect the White House.

Meanwhile, the Federal Bureau of Investigation has an active counterintelligence probe into the scandal, examining whether any Trump campaign official colluded with Moscow.