Russia cannot sustain its air losses for more than a fortnight after at least nine aircraft were shot down in just 24 hours, analysts have said.
Defence analysts said Moscow had been unable to establish air superiority because of its inability – so far at least – to mount "complex" strikes capable of knocking out Ukraine's air force and ground-to-air missile systems.
Western officials have been surprised by Russia's inability to win the battle in the skies given its huge air advantage. Without air superiority, large, slow-moving convoys on the ground can be picked off by Ukrainian drone strikes and shoulder-held anti-tank missiles, supplied to Ukraine by the West.
Military strategists suggest that if Kyiv, the capital, and Kharkiv, in the east, continue to resist the invasion then Vladimir Putin's war effort will falter after "no more than three weeks" unless a major resupply effort is undertaken.
"The Russians have not planned for a long war nor made provisions to sustain it over time," said Prof Lawrence Freedman, emeritus professor of war studies at King's College London.
Russian military losses are being catalogued by observers relying only on visible and verified images, such as a crashed jet or burnt-out tank, posted on social media or recorded by journalists.
The true losses may be higher as a consequence, but verified photographs and footage show that 11 Russian planes, 11 helicopters and two drones have been shot down since the invasion began 12 days ago, including nine over the weekend.
Russian jets destroyed include at least four Su-34 fighter/bomber aircraft, four Su-25 ground attack fighters, two Su-30 fighter jets and nine attack helicopters. The economic cost is huge, estimated at almost a quarter of a billion pounds for warplanes alone.
A US defence intelligence official said in a briefing that emerged overnight: "We continue to observe that the airspace over Ukraine is contested. Ukrainian air and missile defences remain effective and in use. The Ukrainian military continues to fly aircraft and to employ air defence assets.
"Both sides have taken losses to both aircraft and missile defence inventories. We are not going to speak to numbers. We assess that both sides still possess a majority of their air defence systems and capabilities."
The numbers have been catalogued by Stijn Mitzer, a conflict analyst, who has been posting under the Twitter name Oryx and on a website of the same name. The website said: "This list only includes destroyed vehicles and equipment of which photo or videographic evidence is available. Therefore, the amount of equipment destroyed is significantly higher than recorded here."
Defence analysts said Ukraine still had a number of fixed-wing fighter jets operating from airfields in the west of the country and surface-to-air missile sites, some mobile, that Russian forces have been unable to identify and destroy.
The success of surface-to-air missile strikes has forced Russian jets to fly lower, giving them cover from missile attacks but leaving them vulnerable to attack from hand-held portable air-defence systems – guided rockets.
Justin Bronk, research fellow for airpower at the Royal United Services Institute, the UK's leading defence and security think tank, said: "The loss rate becomes unsustainable very fast. If you keep that loss rate up for a couple of weeks, that becomes a real problem for the Russians. They lost nine aircraft over the weekend. That is a lot."
He said Russian cruise missiles had been deployed to hit airfields at the start of the invasion but such weapons, while useful for targeting "fixed infrastructure, hard standings and cratering runways", were less effective at knocking out smaller targets such as fighter aircraft.
He suggested runways could be quickly repaired and fighter jets removed from conflict zones in eastern Ukraine and redistributed in the west of the country ready for deployment. Ukraine had fewer than 100 combat aircraft at the start of the war, and it is unclear what remains after days of conflict.
"Ukraine is still flying," said Mr Bronk who added that the "continued absence" of major air operations mounted by Russia raises "serious questions" about its capability.
He said: "While the early VKS [Russian air force] failure to establish air superiority could be explained by lack of early warning, coordination capacity and sufficient planning time, the continued pattern of activity suggests a more significant conclusion – that the VKS lacks the institutional capacity to plan, brief and fly complex air operations at scale."
He said that if Russia could conduct "complex air operations", Moscow should by now have achieved air superiority by hunting down Ukrainian surface to air missile sites.
After its failure to decapitate Ukraine's leadership in the opening days of the war, experts have warned that Russia is left with the prospect of a humiliating defeat or pressing on with ever greater violence.
In a statement on Monday, Moscow seemed to water down its earlier demands of Ukraine. A Kremlin spokesman said hostilities could stop "in a moment" if Ukraine ceased fighting, amended its constitution to remove any possible future in Nato, acknowledged the loss of Crimea and recognised the separatist areas in the east of the country.
Any mention of "de-Nazifying" Ukraine – seemingly important enough for Russia to have invaded – was dropped.
But defence experts have said any prospect of military defeat or stalemate – which would be seen as a tactical victory for Ukraine – could mean increased violence in the coming days, particularly against civilians.