The failure of the United States and Russia this week to find a diplomatic solution to fizzing tensions over Eastern European security has increased the chance of a new Russian attack on Ukraine even if full-scale war is far from inevitable, analysts say.
Russia stands accused of having massed some 100,000 troops close to the Ukrainian border, as it seeks a commitment from the West that the pro-EU former Soviet state will never join NATO.
Senior Russian and US officials this week held talks in Geneva but there was no hint of any breakthrough, with Washington warning by the end of the week Moscow could stage a false flag operation within weeks to precipitate an invasion.
"We have information that indicates Russia has already prepositioned a group of operatives to conduct a false-flag operation in eastern Ukraine," said Jen Psaki, the White House press secretary.
"The operatives are trained in urban warfare and in using explosives to carry out acts of sabotage against Russia's own proxy forces."
Ukraine was meanwhile hit by a blistering cyberattack in the early hours of Friday blamed by the West on Russia and which some analysts feared could be the prelude to an attack.
Moscow responded to the ousting of Ukraine's former pro-Russia president in 2014 by seizing in a lightning military operation the Ukrainian Black Sea peninsula of Crimea, an annexation never recognised by the international community.
It then backed separatists who took control of two eastern Ukrainian regions in an unsolved conflict that has left over 13,000 dead.
- Is there a way out? -
NATO has made clear that it will never rule out the possibility that Ukraine could join the alliance, even if analysts see this as a very remote chance for now.
"Regrettably war is more likely. We've reached a real impasse," said Melinda Haring, deputy director of the US-based Atlantic Council's Eurasia Center. "As things stand, the Russian and US positions are irreconcilable."
Dmitry Trenin, director of the Carnegie Moscow Center, added: "The absence of a diplomatic solution logically leads to a further exacerbation of this crisis with a military solution already seen as a way out."
Samuel Charap, senior political scientist at the RAND Corporation, noted that NATO had not even offered Ukraine a membership action plan let alone membership.
"Finding a way to say that, that is politically viable, is going to be very difficult, of course, but so will a war," he said.
- What is Russia planning? -
But analysts say despite the ominous signals it is impossible to read the mind of Russian President Vladimir Putin.
For Marie Dumoulin, director of the Wider Europe programme at the European Council on Foreign Relations (ECFR), "the decision on whether or not to continue these talks will be taken by Vladimir Putin and no-one knows right now what it will be".
Francois Heisbourg, special advisor to the Foundation for Strategic Research (FRS) in Paris, said "the situation is totally volatile" and the "risk of war high".
Charap said it was "an open question" whether the talks with the US had simply been a "time buying exercise" for Russia.
"And I don't know whether that's the case and I don't think any of us really know."
- What shape would military action take? -
Even if Russia opted for military action, it may not come down to a full-scale invasion of Ukraine.
Putin could look to responses such as deploying Russian missiles in the separatist-controlled Donbass region of eastern Ukraine or Crimea, according to the prominent Russian security analyst Maxim Suchkov.
Heisbourg argued another option could be limited "territorial gains" that would link the Donbass region with Crimea.
US intelligence sources have indicated that no final decision has been taken by Russia. A French diplomatic source, who asked not to be named added: "I do not believe there are preparations for an immediate invasion."
- What would the consequences be? -
Russia has repeatedly been warned by Western powers of "massive" consequences were it to attack Ukraine again, although these would likely take the form of sanctions rather than any military riposte.
Moscow and the West would suffer a crisis unprecedented since the Cold War, likely tightening further the growing Russian partnership with China as well as its alliance with the autocratic regime of Belarus which borders three EU states.
For Dumoulin, "the Russians are blowing hot and cold" and keeping "maximum pressure to obtain more" concessions from the West over the architecture of European security in former Communist bloc states.
"The scenario of military intervention is not the most probable" because its "cost, military, political, financial and human" would be considerable, she said.
Haring argued Putin would strike Ukraine "in a way short of war", prompting Europe "to wring its hands but stop short of unleashing its harsh package of sanctions."