Russia has consistently denied sending tanks into Ukraine, arguing that any vehicles used by separatist forces there must have been captured from the Ukrainian army itself.
But now experts at the International Institute for Strategic Studies (IISS) in London have told the BBC that they have identified a Russian tank in a separatist column in eastern Ukraine that they say could only have come from across the border in Russia.
Whilst date and location are unconfirmed, he says the operator of the convoy is apparent: flags associated with the separatist movement are clearly displayed and some vehicles feature bright green areas, a common feature of separatist armour.
The mixed convoy includes at least three T-72B1 Main Battle Tanks (MBTs) but it is the appearance of a lone, more modern T-72 variant (shown below) that is of particular significance.
This variant, distinguished by the prominent Kontakt-5 Explosive Reactive Armour (ERA) arrangement - the boxes on the turret front - is commonly referred to by Western sources as the T-72BM.
It is operated by the Russian Army in large numbers, but crucially it is not known to have been exported or operated outside of Russia.
According to the IISS, the presence of this variant in Ukraine therefore strongly supports the contention that Russia is supplying arms to separatist forces.
Up to now deniability has been a key-note of Moscow's "ambiguous war" strategy in eastern Ukraine.
The IISS says that the first separatist tanks observed were identified as T-64BV, a variant constituting the majority of the Ukrainian army fleet.
It was therefore initially assumed that they, like separatist armoured vehicles before, had been captured in engagements with government forces or through access to known army depots within contested areas.
However, on 14 June Nato published strong evidence that these tanks had been supplied by Russia.
Although the T-64BV was withdrawn from Russian active service, the IISS assesses that a significant quantity remain in storage.
It is therefore feasible that a number of T-64BV, surplus to current Russian requirements, could have been supplied to the separatist forces.
Mr Dempsey notes that "the Soviet-era tanks operated by the separatists have until now represented those that could have been potentially acquired internally within Ukraine, providing a degree of plausible deniability to any suspected third-party supplier."
The most recent separatist MBT variant observed, however, is assessed to have been operated only by the Russian Army," he adds.
The IISS analysis underscores the shaky foundations of Russia's constant insistence that it is not involved in the fighting.
This taken together with Nato satellite imagery and the recent interviews with Russian servicemen captured inside Ukraine suggests a very different reality from that presented by Moscow.