The expertly armed, masked men in matching camouflage stripped of all insignia are tough, taciturn and tactically devastating.
And according to President Vladimir Putin, they are not -- absolutely, categorically not -- elite Russian special forces.
So who are the members of this mysterious military or paramilitary force operating in eastern Ukraine, nicknamed "little green men" by many here?
For Kiev and its Western backers, the units, observed moving in lightning-fast and cohesive team formation, are indisputably Russian commandos sent by Moscow to sow trouble, no matter what Putin says. They also appear to be almost identical to those who operated in Crimea before the peninsula's annexation by Russia last month.
For separatist insurgents whose fealty lies with the Kremlin, they are simply preternaturally good examples of their rag-tag, homegrown "self-defence volunteer brigades".
In many of the 10 towns in the east controlled by the rebels, the fearsome fighters guard seized public buildings and offer few, if any, words to the curious.
They do, however, easily stand out from their less disciplined brothers in arms.
They wear camouflage uniforms, black ski masks, sports shoes and bullet-proof vests. That is a far cry from the hodge-podge of military surplus and camping attire thrown on by the ordinary separatists -- who are also more likely to chat proudly.
On Wednesday, when the Ukrainian military sent an armoured column to confront the rebels, it was the (not so) "little green men" who sprang into action after the vehicles were stopped by angry locals.
The capable fighters quickly took control of six of the armoured personnel carriers and drove them into the centre of the town of Slavyansk -- where one of them displayed his mastery of the tracked machine by sending it spinning in high-speed doughnuts.
The same day, when another armoured column was stopped by a crowd and agreed to render its weapons inoperable in return for being allowed to return home, three of the professionals stepped forward to order the locals into a corridor providing an orderly exit worthy of a parade ground.
- 'Tell-tale' military training -
For expert observers, the mysterious fighters' cover is blown.
The military commander of NATO, US General Philip Breedlove, enumerated on his official blog why he -- and the leaders of the US, France, Britain and Germany -- believed the men could only be Russian special forces.
As well as their "tell-tale military training and equipment" and team tactics, they handled their rifles like pros with the muzzles pointed down and the fingers off the triggers when not in combat mode, he wrote.
Their "coordinated use of tear gas and stun grenades" during the take-over of Ukrainian public buildings were well beyond the ken of a nascent militia, as was their discipline at checkpoints.
"Finally, the weapons and equipment they carry are primarily Russian army issue. This is not the kind of equipment that civilians would be likely to be able to get their hands on in large numbers," Breedlove said.
Olexiy Melnyk, a military expert in Kiev's Razumovka Centre, agreed.
"These green men look very much like those who were at work in Crimea and yet again Vladimir Putin doesn't acknowledge there are Russians," he said.
Putin had long denied that the Russian military was deployed in Crimea -- but on Thursday he reversed course to admit that in fact Russian army units had been there "behind the Crimean self-defence forces" on a mission to "protect" the local population.