A foreign policy adviser to the Kremlin, in a rare break from Moscow, has sounded the alarm on the "incomprehensible" and "very embarrassing" Russian invasion of Ukraine.

"I'm trying to keep going; I'm trying to continue doing what I'm doing, but of course I'm depressed," Russian International Affairs Council Director-General Andrey Kortunov told Sky News in an exclusive interview. "All of us are depressed.

"And I think that it's very embarrassing for all us – not only because we turned out to be wrong, but also because Russia and all of Russians will be in difficult position in many countries, not even only western countries but all over the place for a long time."

Words of disloyalty to Vladimir Putin and dissenting opinions around the Kremlin are extraordinarily rare – if not dangerous for the whistleblower – but Kortunov has broken ranks to say he "cannot comprehend" the rationale for Putin's "special military operation" that has become a bloody, full-scale invasion of its neighbor.

"I was shocked because for a long time, I thought that a military operation was not feasible," Kortunov added to Sky News. "It was not plausible.

"My advice today, given the current situation on the ground, would be to turn a ceasefire into the top priority. We have to stop the conflict.

"To start with, we have to get to the negotiating table not just with Ukraine, but also with the West.

Kortunov is on a foreign policy panel that advises Putin and their briefing papers have reportedly been delivered by ignored.

"We tend to believe that the name of the game is development, but I can imagine that some people around Mr. Putin believe that the name of the game is survival," Kortunov continued.

Putin reportedly is listening to a very select group of advisers in his inner circle, including "securocrats known as siloviki": generals, spies, and friends.

"I think that there are many people in the Kremlin who should be depressed because the price will be substantial and of course, you know, we should be depressed also because people are being killed," Kortunov added. "You know, it's something that we should never forget."

Others in Russia have condemned the invasion, including scientists and journalists, risking fines up to $8,300, if not prison under laws passed in 2012, according to the Daily Mail.

An open letter from 6,100 academics, scientific journalists and medics in Russia told the Kremlin there was no "rational justification for this war," which will make the country a "pariah" and "doomed to isolation."

They called for "an immediate halt to all military operations directed against Ukraine," while imprisoned Russian dissident Alexei Navalny, 45, called Putin an "obviously insane czar," according to the Mail.

"We cannot wait even a day longer," Navalny said, according to a tweeted statement from his spokesman. "Wherever you are. In Russia, Belarus or on the other side of the planet. Go out onto the main square of your city every weekday at 19.00 and at 14.00 at weekends and on holidays."

"Let us at least not become a nation of frightened silent people," he added. "Of cowards who pretend not to notice the aggressive war against Ukraine unleashed by our obviously insane tsar."

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