The precise origins of the rebels are unknown, although the West has repeatedly blamed Russia for instigating their recent activities.
Read MorePutin risks upstaging talks on calming Ukraine crisis
On Thursday, Russian President Vladimir Putin admitted for the first time that Russian soldiers were present in Crimea before its annexation by Russia, but denied that forces were currently on the ground in eastern Ukraine.
However, Olexander Scherba, Ukraine's ambassador-at-large, told CNBC earlier this week that it was "very obvious" that Russian forces were in the country.
Analysts agreed. "There are many uncertainties surrounding exactly who these people are, but what is patently clear is that they are Russian agent provocateurs," Nicholas Spiro of Spiro Sovereign Strategy told CNBC.
The rebels do not wear any obvious Russian military insignia, but this could be because any signs of Moscow endorsement could increase the chance of the West instigating more severe sanctions.
The West has already threatened further penalties for Russia, beyond the current travel bans and asset freezes on a number of Ukrainian and Russian officials. But more extreme sanctions, aimed at trade and the energy sector for instance, are unlikely to materialize unless Russia actually invades Ukraine.
"It's not in Russia's interests to go in all guns blazing," he said. "Instead they're stirring things up, destabilizing things. These are Soviet-style tactics – and this eastern part of Ukraine is fertile ground for these types of tactics."
Read MoreRussia's economy feels the sting of Ukraine crisis
Duncan Leitch, a consultant who worked in east Ukraine for 15 years and is now an academic at the Centre for Russian and East European Studies at the U.K.'s Birmingham University, agreed.
"To what extent these Russians have been officially sent over the border, or have come over voluntarily is unknown," he said. "What is known is that they've been encouraging the Ukrainian people to demonstrate again the interim government."
Russia's aim, according to Leitch, is to provoke a reaction from the Ukranian government, which could further undermine its control of the country.
"One analysis is that Moscow wants to create a pre-text for a Russian incursion," Leitch warned.
It is unlikely, however, that the soldiers occupying buildings across the east of Ukraine, in cities including Donetsk and Slavyansk, are all be Russian.