Deadly clashes in eastern Ukraine have spiked fears of all-out war in the region, as key talks about a resolution get underway between the European Union (EU), U.S., Ukraine and Russia.
Three pro-Russian separatists were killed overnight at Mariupol military base, about 40 kilometers from the Russian border, according to Ukraine's interior minister. Armed rebels continue to occupy government buildings in towns and cities across eastern Ukraine.
But who are these armed, flag-waving rebels, who seem so keen to follow in Crimea's footsteps and break away from Ukraine?
The precise origins of the rebels are unknown, although the West has repeatedly blamed Russia for instigating their recent activities.
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."
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.
"There is genuine pro-Russian sentiment in these regions, and whilst the action is being orchestrated by Russia, it's definitely picking up on more grass-roots level," Andrew Foxall, director of the U.K.'s Russia Studies Centre, told CNBC.
Eastern Ukraine was the stronghold of the country's ousted pro-Russian president, Viktor Yanukovych. It also has strong economic ties with Russia, giving it an incentive to remain close to its neighbor.
"The economic ties are very strong; many eastern Ukrainians have family in Russia," Leitch said.
The region is the industrial heartland of Ukraine and its factories produce many agricultural and industrial goods that the West would view as sub-standard, but which are snapped up by Russia.
According to Leitch, Russian-money is also behind many of the factories in the region's cities, and the flow of workers across the border both ways has always been prevalent.
In addition, many eastern Ukrainians speak Russian and feel culturally closer to Russia that the rest of Ukraine.
A number of ethnic Russians in eastern Ukraine have alleged harassment by Ukrainian nationalists – indeed, this was one of Russia's justifications for the annexation of Crimea in February.
However, earlier this week, the United Nations said that although there were some attacks against the ethnic-Russian community, "These were neither systematic nor widespread."
Leitch stressed that the language/ethnicity divide in east Ukraine was overblown.
"My judgement is that most Ukrainians – including those in the east – are loyal to the idea of an independent sovereign Ukrainian state," he said.
"Those on the streets have been radicalized. They've been told that the interim government is fascist and intends to strip away their rights to their language. That's why they're out there," he added.
Foxall agreed. "Because the east is closer to Russia – and the flow of people over the border is far more intense - they're much more exposed to the propaganda regime."