Three former Obama administration officials, who spoke on the condition of anonymity in order to be able to divulge the private conversation, said that during an Oval Office meeting shortly after the 2016 election, Obama directly warned Trump against appointing Flynn to his cabinet. The information suggests that concerns over Flynn's background rose to the highest level of government.

A number of congressional committees launched investigations after US intelligence agencies concluded that Russian President Vladimir Putin had ordered hacking of Democratic political groups to try to sway the 2016 election toward Trump. Flynn has emerged as a central figure in those probes.

Moscow denies any such interference.

Precarious timing

The news about Obama's warning to Trump came just hours before the highly anticipated hearing before a Senate Judiciary subcommittee of former acting Attorney General Sally Yates as part of the investigation into alleged Russian interference in the presidential election.

In her first appearance on Capitol Hill since being fired in January just ten days into office, Yates recounted how in late January she told White House counsel Don McGahn that Flynn was not telling the truth about his contacts and warned about potential blackmail coming from Russia. 

"We believed that Gen. Flynn was compromised," Yates told members of the Senate. "To state the obvious, you don't want your national security adviser compromised with the Russians."

Despite this, the White House took 18 days to dismiss Flynn - drawing criticism from high-ranking Democrats, who have articulated concerns about connections between the Trump administration and Moscow.

Yates, a longtime federal prosecutor and Obama administration appointee, was fired by Trump on January 31 after refusing to defend the administration's travel ban on certain Muslim countries.

Yates and former Director of National Intelligence James Clapper, who also testified on Monday, said they didn't know how journalists got ahold of classified information about Flynn's conversations with the Russian ambassador. Earlier that day, Trump seemed to suggest on Twitter that Yates had a hand in the leaks.

Following her testimony, the president used the social media platform once more to dismiss Yates' statements, claiming that Russian involvement in the presidential election was a hoax despite there being several active investigations ongoing at multiple levels of government.

Following the hearing, subcommittee chairman Lindsey Graham, a staunch opponent of Trump during the 2016 presidential race, praised Yates but said he saw no problems with how the White House handled the Flynn issue. "All I do know is that Gen. Flynn got fired," he said. "To me that was the responsible way forward. Should it have been 15 days? Should it have been one day? To me that's not a very big concern."

ss,kbd,bc/kl (AFP, AP, Reuters)