The prospect of Russia's Federal Security Service (FSB) launching a coup against President Vladimir Putin is growing every week, according to a Russian intelligence source. 

British newspaper The Times, citing exiled Russian activist Vladimir Osechkin, reports a whistleblower "at the heart of Russian intelligence" has exposed discontent within Russia's security services over the stymied invasion of Ukraine. 

Osechkin has landed himself on the Russian Government's most wanted list for his previous work exposing abuse in the country's prisons. 

The information he has shared from the Russian intelligence agent comes in the form of almost a dozen letters. 

A Ukrainian flag is seen on top of a checkpoint as black smoke rises from a fuel storage of the Ukrainian army following a Russian attack, on the outskirts of Kyiv on March 25. Photo / AP 

Osechkin told The Times much of the discontent among FSB agents stemmed from the effects of Western sanctions, which mean they will no longer be able to "go on holidays to their villas in Italy and take their kids to Disneyland Paris". 

"For 20 years, Putin created stability in Russia. FSB officers, policemen, state prosecutors – those people inside the system – were able to live good lives," he said. 

"But now that has all gone. They recognise that this war is a catastrophe for the economy, for humanity. They don't want to go back to the Soviet Union. 

Ukrainian supporters demonstrate in Trafalgar Square in London on March 24 as they protest against the Russian invasion of Ukraine. Photo / AP 

"For every week and every month that this war continues, the possibility of a rebellion by those in the security services increases." 

Osechkin said his source was taking "a very big risk" by sending him letters, given the Putin regime's long-running, brutal intolerance for dissent. 

This comes amid the latest assessment from British intelligence, which believes Russia has suffered such "considerable losses" in Ukraine that it is now looking to mobilise its reservist and conscript manpower, along with private military companies and foreign mercenaries. 

"It is unclear how these groups will integrate into the Russian ground forces in Ukraine, and the impact they will have on combat effectiveness," UK intelligence said today. 

Britain also floated the possibility that Russian troops are "becoming encircled" around Kyiv, having failed in their initial objective to encircle the city themselves. 

'High risk of escalation' 

Meanwhile, there are growing fears a breakdown in communication between the United States and Russia could intensify the threat of the conflict expanding beyond Ukraine. 

A new report has revealed military leaders in the US have been unable to contact their Russian counterparts since the invasion began in late February, leaving the world's two largest nuclear powers under an information cloud. 

Analysts from the US warned even a single incident or miscalculation from a soldier on the ground could spark a devastating butterfly effect. 

Former supreme allied commander of Nato James Stavridis highlighted the fact the majority of those fighting were still incredibly young and could potentially spark a much bigger catastrophe with one misplaced missile. 

This risk is worse because of the lack of communication between US and Russian leadership. 

"There is a high risk of escalation without the firebreak of direct contact between the most senior officials," Admiral Stavridis, who filled the position from 2009 to 2013, said via the Washington Post. 

"Very young people are flying in jets, operating warships, and conducting combat operations in the Ukrainian war. They are not seasoned diplomats, and their actions in the heat of operations can be misunderstood." 

Admiral Stavridis warned the communications blackout would only make things worse over time, with US leaders now in the dark and left to play guessing games over Russia's true intentions. 

Holes from Russian artillery shelling are seen on the wall of a burning multi-storey apartment following an attack in Kharkiv, Ukraine on March 25. Photo / AP 

He said during his time in the military hot seat, a line was always open to some form of Russian authority, revealing he rang Moscow "on several occasions to clarify a situation and de-escalate". 

"We must avoid a scenario of Nato and Russia sleepwalking into war because senior leaders can't pick up a phone and explain to each other what is happening," he said. 

Tensions were stoked again last week when Russian missile strikes killed dozens of Ukrainians near the Polish border. As a Nato member, Poland has strongly opposed the invasion and has made its own moves against Putin, expelling 45 Russian diplomats for espionage on Wednesday. 

Lena Danilova, 39, walks with her daughter Kira, 2, in the Podolskyi neighbourhood in Kyiv, Ukraine yesterday. Danilova is having second thoughts about staying as the war drags on. Photo / AP 

According to Admiral Stavridis, if a Russian attack was to hit a Nato-occupied facility, a "nightmare scenario" would follow. 

"A nightmare scenario would be a Russian missile or attack aircraft that destroys a US command post across the Polish-Ukrainian border," he said. 

"A local commander might respond immediately, thinking the event was a precursor to a wider attack. This could lead to rapid and irreversible escalation, to include potential use of nuclear weapons." 

'Defend every inch of Nato territory' 

Rob Lee of the US Foreign Policy Research Institute said the risk of a widespread international conflict sprouting from the Ukraine war was of serious concern. 

"The risks are obviously elevated currently," he said. 

"Russia is striking targets in western Ukraine, which are not far from the border with Nato members, and the Ukrainian Air Force apparently continues to operate from that region, which means there is a risk that its aircraft could be mistaken for Nato aircraft across the border." 

US President Joe Biden staunchly opposed sending US troops to Ukraine, but ominously warned it would "defend every inch of Nato territory", some of which is perilously close to the Ukrainian border. 

Ukrainian officials, including President Zelenskyy and Foreign Minister Dmytro Kuleba, have been criticised for urging Western powers to come to the nation's defence on the ground, with many quick to point out it could mark the beginning of World War III. 

With superpowers China and India yet to officially condemn Russia's territorial move, the playing field appears shaky at best. 

Peace talks between Kyiv and Moscow continue to drag on with no solid compromise met between the two nations. Zelenskyy has already conceded his country may never join Nato, which Putin has cited as one of his reasons to invade. 

"If there's just 1 per cent chance for us to stop this war, I think that we need to take this chance. We need to do that. I can tell you about the result of this negotiations – in any case, we are losing people on a daily basis, innocent people on the ground," Zelenskyy said earlier this week. 

"Russian forces have come to exterminate us, to kill us. And we can demonstrate that the dignity of our people and our army that we are able to deal a powerful blow, we are able to strike back. But unfortunately, our dignity is not going to preserve the lives. 

"I think that we have to use any format, any chance in order to have a possibility of negotiating, possibility of talking to Putin. But if these attempts fail, that would mean that this is a third World War."

- by Sam Clench and Alex Blair, news.com.au