Russia's invasion of Ukraine has resulted in unforeseen obstacles and challenges that make victory a near impossibility as casualties continue to mount amid crippling sanctions, according to a letter purportedly authored by a Russian intelligence analyst in one of Moscow's security agencies. 

The more than 2,000-word report by a whistleblower in the Federal Security Service of the Russian Federation (FSB), the successor agency to the KGB, said the number of Russians killed in the conflict could number 10,000, a figure in line with Ukrainian estimates and in stark contrast to the 498 reported by the Kremlin last week. 

The report said Russia doesn't have an accurate number of its death count after losing contact with military units inside the country. 

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People look at the gutted remains of Russian military vehicles on a road in the town of Bucha, close to the capital Kyiv, Ukraine on March 1. Russian forces have struggled to make progress in Ukraine amid heavy fighting and fierce resistance.  (AP Photo/Serhii Nuzhnenko)

The author said the FSB was being blamed for Moscow's failures in fending off Ukrainian defense forces, while the agency was not given advance warning about the invasion. Russia was also unprepared to deal with the effects of the global sanctions it now faces as the ruble continues to plunge, the letter said. 

"No one knew that there would be such a war, so no one prepared for such sanctions," the report said. "It’s just that there is no option for a possible victory."

Meanwhile, officers were pressured to tailor reports to the narrative desired by Russian leaders. 

"We have been increasingly pressed to customize reports to the requirements of management - I once touched on this topic," the author claims. "All these political consultants, politicians and their retinue, influence teams - all this created chaos."

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"Now, even if Zelensky is killed, taken prisoner, nothing will change," the report said. "And now even those who were loyal to us are against it. Because it was planned from above, because we were told that there would be no such option, unless we were attacked.

"Even with minimum resistance from the Ukrainians we'd need over 500,000 people, not including supply and logistics workers," the author claims.

The letter also compared Russia's failure to conquer Ukraine with the mistake made by Nazi Germany in World War II. The author said it remains to be seen whether leaders will start a conflict with Europe in an effort to reduce the sanctions. 

"Now I do not rule out that then we will be drawn into a real international conflict, like (former Nazi German leader Adolf) Hitler in 1939. And then our (pro-Russian) Z (symbol) will be compared to us with a swastika."

Vladimir Osechkin, founder of the Gulagu.net group, published the report on his Facebook page last week. Christo Grozev, who works for the Netherlands-based investigative journalism group Bellingcat, tweeted that he showed the letter to two FSB contacts, one current and one former employee. They had no doubt that the author was a colleague, he said. 

Grozev was skeptical about its authenticity but said the "letter appeared different" in that it came from a reputable source and was "way longer than a forger would choose to make it."

Still, the letter could be part of a disinformation attempt from Ukraine, said Rebekah Koffler, a former Defense Intelligence Agency officer and author of "Putin's Playbook: Russia's Secret Plan to Defeat America." She cited several details in the letter that appeared to be suspicious. 

"Both sides right now are waging information warfare on each other," she told Fox News. "It's very long and incorporates all kinds of things. If he's really an FSB officer in the middle of this conflict right now, who has the time to write such a long post that would incorporate all these various things?"

She cited the author's reference to Hitler as one example. 

"The Hitler thing. This is a Western narrative," she added. "The Russians hate Hitler, but it's unlikely for them to compare Putin with Hitler."

The author of the letter addressed whether Putin would initiate a nuclear strike against the West. 

"Is there a possibility of a local nuclear strike? Yes. Not for military purposes (it will not give anything - this is a defense breakthrough weapon), but with the aim of intimidating others," the report said. 

He added that the SVR, Russia's foreign intelligence service, was "digging the earth" for evidence that Ukraine has secretly created nuclear weapons to justify such a strike. 

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"From the cynical, I will only add that I do not believe that VV Putin will press the red button to destroy the whole world," the author states.