The soldiers who took control of Crimea without firing a single shot are likely to be Russian special forces, airborne units, and possibly commandos from the elite Spetsnaz force, London-based military experts say.
Igor Sutyagin, a specialist on Russia at RUSI (Royal United Services Institute), said the soldiers in unbadged uniforms who have fanned out across Crimea were using kit only handled by elite divisions.
"The units which blockaded the coastal defence units at the weekend; that was Spetsnaz," Sutyagin told AFP on Monday.
"They are recognisable because they have their specialist equipment. It might be the 3rd Spetsnaz brigade from Tolyatti," a city in Russia's Samara Oblast region, he added.
The crack fighters are "experts in deep, deep reconnaissance and sabotage" and are being supported by "at least two marine brigades from the Black Sea and Baltic Sea," according to Sutyagin.
Matthew Clements from Jane's Intelligence Review agreed that special forces had been active in the southern Ukraine peninsular.
"There has been an effort from the Russian forces in Crimea to reduce any kind of identification," he said.
"But what we do know is that there are certainly units of special forces and airborne units that have been sent to Crimea."
A former high-ranking European security official, speaking on condition of anonymity, said he was "200 percent" sure that the operation bore all the hallmarks of Spetsnaz fighters, describing them as experts in creating "confusion and chaos in the framework of a psychological offensive."
The analysts spoke as Ukrainian border guards described how Russian troops and military planes were flowing into Crimea in violation of international agreements.
Insignia revealing nationality, unit and rank are absent from their uniforms while many are wearing ski masks.
Armed with Kalashnikovs, the soldiers have taken up positions around strategic locations including airports, government buildings and Ukrainian army barracks.
Military transport planes, helicopters and warships have all been sent to the region.
Silence is the rule when it comes to Spetsnaz GRU, which takes orders from military intelligence services, and their actions are shrouded in secrecy, much like the SAS special forces in Britain.
Spetsnaz operatives are specialists in sabotage, assassination and capturing strategic targets. They are regularly dispatched to scout behind enemy lines, in uniform or civilian clothes, often in highly dangerous conditions.
They are the successors of the NKVD groups which carried out raids against the Nazis during World War II and have been at the heart of all the major conflicts Russia has been involved in since then. Russia praised their deployment in Afghanistan and Georgia.
However, rights group Amnesty International has accused them of committing crimes against humanity during the war in Chechnya.
They played a major role in securing the release of hostages during the storming of Moscow's Dubrovka Theatre by Chechen rebels, and the 2004 siege of a school in Beslan, North Ossetia.
In both cases, the standoffs came to a bloody conclusion.