Kremlin officials are becoming "increasingly" worried Vladimir Putin could use limited nuclear weapons as part of Russia's invasion of Ukraine.
Officials at high levels of government spoke to Bloomberg, claiming they believed the invasion was a "catastrophic" mistake which will set Russia back years.
Speaking on condition of anonymity, the Kremlin sources said there is no sign Putin will change his mind and faces no direct challenge within the walls of the Kremlin.
He has also repeatedly ignored those who have attempted to warn him of the crushing effects his war could have on Russia's economic and political future, destroying two decades of growth.
Almost overnight, Western nations implemented biting sanctions, cutting off half of the central bank's $640bn reserves.
Some said they share fears voiced by the US that Putin will turn to a limited nuclear war, in which two nuclear superpowers engage in a direct confrontation which does not end in either a surrender or massive destruction.
However, US officials have clarified there is "no practical evidence" nuclear weapons will be used imminently.
Watch: Ukrainian forces repel Russian troops in east of the country
On Thursday the director of the CIA said Putin could turn to using a tactical or low-yield nuclear weapon out of "potential desperation” to claim a small victory.
William J Burns — who was formerly the US ambassador to Russia — said: "Given the potential desperation of President Putin and the Russian leadership, given the setbacks that they’ve faced so far, militarily, none of us can take lightly the threat posed by a potential resort to tactical nuclear weapons or low-yield nuclear weapons."
But he cautioned he had yet to see any "practical evidence" of military deployments which would indicate this move was imminent.
Fears were further stoked on Tuesday after Russian foreign minister Sergei Lavrov said his country was not considering using nuclear weapons "at this stage" of the invasion.
Speaking to India Today, Lavrov was asked if Russia was looking at deploying part of their nuclear arsenal.
He said: "At this stage, we are considering the option of conventional weapons only," according to Russia's RIA state news agency.
Despite Western claims Putin had thought he could take Ukraine within days of mounting an invasion, it has now been seven weeks since Putin ordered his troops in on 24 February.
But having failed to take control of Kyiv, Russian forces have completely withdrawn from the north of Ukraine and have instead focused on a takeover of the Donbas region in the south east.
Russia has hinted at the use of nuclear weapons in the past, and while no nuclear arsenal has been deployed, Putin's forces have been accused of using chemical weapons in Ukraine.
Russia's actions have promoted a potential expansion of Nato, with Finland and Sweden announcing their intentions to join in the summer.
Putin has long harboured animosity towards the peacetime alliance and says he regards it as a direct threat.
Dmitry Polyanskiy, the Russian deputy ambassador to the UN, warning Moscow reserved the right to deploy nuclear weapons if "provoked".
Asked if Putin was right to hold the prospect of nuclear war over the rest of the world, Polyanskiy told Sky News: "If Russia is provoked by Nato, if Russia is attacked by Nato, why not, we are a nuclear power.
Watch: Fearful Russian support war against 'satanists' in Ukraine
"I don’t think it’s the right thing to be saying. But it’s not the right thing to threaten Russia, and to try to interfere.
"So when you’re dealing with a nuclear power, of course, you have to calculate all the possible outcomes of your behaviour."
Fears of a nuclear disaster were stoked after Russian troops took over Chernobyl nuclear station.
In March, Putin's forces began shelling Europe’s largest nuclear power plant –– Zaporizhzhia — as part of its assault on Ukraine.