Washington (CNN) Former acting Attorney General Sally Yates on Friday slammed President Donald Trump's "inexplicable refusal" to confirm Russian interference in the 2016 election, saying it hinders the US Intelligence Community.

As the highly anticipated , first ever Trump-Putin meeting was getting underway, Yates, a career Department of Justice official who was fired by Trump in January, tweeted: "POTUS' inexplicable refusal to confirm Russian election interference insults career intel pros & hinders our ability to prevent in future."

Later Friday, however, Secretary of State Rex Tillerson told reporters that Trump opened his meeting with Putin by "raising the concerns of the American people regarding Russian interference in the 2016 election."

"They had a very robust and lengthy exchange on the subject," Tillerson added.

But when he spoke at a news conference in Warsaw, Poland, on Thursday, Trump only offered a tepid endorsement of findings by intelligence officials that Russia was behind the election meddling.

"I think it very well could be Russia but I think it could very well have been other countries," Trump said. After noting "wrong" intelligence that led to the "one big mess" in Iraq, he concluded, "Nobody really knows. Nobody really knows for sure" who did the hacking.

Former Director of National Intelligence James Clapper, appearing on CNN's "The Situation Room" Thursday, attacked the claim that others may have interfered.

"As far as others doing this, boy, that's news to me," Clapper said. "We saw no evidence whatsoever that it was anyone involved in this other than the Russians."

JUST WATCHED Yates: Flynn vulnerable to blackmail Replay More Videos ... MUST WATCH Yates: Flynn vulnerable to blackmail 01:57

Yates has voiced concerns about the Trump administration and Russia before. In a May congressional hearing about the firing of former national security adviser Michael Flynn, Yates testified: "To state the obvious: You don't want your national security adviser compromised with the Russians."

Trump fired Yates in January, citing her decision to not defend Trump's travel ban. At the time, the White House criticized Yates for "betray(ing) the Department of Justice."

The tweet was Yates' second as a private citizen since she launched her account last month.