Sen. Bernie Sanders Bernie SandersSenate Republicans signal openness to working with Biden Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Schumer, Sanders call for Senate panel to address election security MORE (I-Vt.) says he is unnerved by reports President Trump’s national security adviser Michael Flynn talked sanctions with a Russian official before formally starting his post.

This is a very troubling development,” he said Friday on CNN’s “OutFront.” “This is very, very troubling and I think the president is going to have to tell us what he’s going to do about it.”

Sanders added the allegations surrounding Flynn are the latest concerning link between the Trump administration and Russia.

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“It speaks to the broader issue of Russia’s involvement in our elections,” said Sanders, who was a 2016 Democratic presidential candidate.

“It speaks to the issue of President Trump being buddy-buddy with a thug and murderer who is the head of Russia,” he added, referencing Russian President Vladimir Putin.

“And now we’re learning there may have been discussions between Flynn and the Russians before this administration came to power.”

Reports emerged Thursday that Flynn spoke with Russian Ambassador Sergey Kislyak about sanctions in December.

Current and former U.S. officials confirmed the alleged exchange, seeming at odds with Flynn’s assertion the pair never broached the subject with Kislyak before Trump’s Jan. 20 inauguration.

Flynn’s spokesman on Thursday downplayed his denial, stating that “while he had no recollection of discussing sanctions, he couldn’t be certain that the topic never came up.”

The Obama administration imposed fresh sanctions against Russia in December following revelations the Kremlin tried influencing the 2016 presidential election in Trump’s favor.

Trump administration officials have denied that Flynn and Kislyak spoke about sanctions in phone calls before the president entered office.

Two U.S. officials told the Washington Post that Flynn led Kislyak to believe that the sanctions would be reevaluated once Trump became president.

“Kislyak was left with the impression that the sanctions would be revisited at a later time,” one official said.

Trump on Friday said "I don't know about that" when asked about the reports dogging Flynn, adding he would investigate the matter further.