Battalion Vostok - or the East Battalion - is a heavily armed, well-organised fighting group that has burst onto the scene in Ukraine's Russian-speaking east and appears to be seeking to lead the fight to prise the region from Kiev and merge it with Russia.
The group encountered at the former Ukrainian base included a total of at least five fighters from the Russian Caucasus region of North Ossetia and from a Russian-backed enclave of Georgia.
They acknowledged they had been fighting alongside Chechens from Russia's former rebellious region of Chechnya, but these, they said, had now gone home.
The presence of fighters from Russia and other parts of former Soviet space is likely to feature prominently in talks this week when Ukraine's President-elect, Petro Poroshenko, meets U.S. President Barack Obama and later, possibly, Russia's Vladimir Putin.
"The split of the country is final. There is nothing uniting us with them (the Kiev leadership) now," Alexander Khodakovsky, a defector from the Ukrainian state security service who now commands Battalion Vostok, told Reuters.
"Kiev has already understood that they have lost south-eastern Ukraine, that it is a sphere of Russian influence, and one way or another it will remain so," said the 41-year-old.
The men of Battalion Vostok see Russia as the heart of their own civilisation and values, irreconcilable with the pro-Western course taken by the Ukrainian authorities.
Kiev denounces them as terrorists and accuses Russia of supporting the rebellion in the east, where separatists have proclaimed independent "people's republics" and where scores of rebel fighters died in clashes with Ukraine's army in May.
Composed and relaxed, Khodakovsky strikes a sharp contrast with many of his men - taciturn and hiding their faces behind balaclavas - who fought the heaviest battles with the Ukrainian army around Donetsk city airport and in the village of Karlovka this month.
For him, the conflict is part of a global standoff between Russia and the United States in a bipolar world, more than two decades after Ukraine took its independence during the collapse of the Soviet Union.
"Technically, we have been an independent country, but the borders were only a formality, and we could go on feeling ourselves part of Russia," he said.
"We do not want to be against Russia, and if we are allied with the European Union and NATO, we would automatically be. We made our choice."
Moscow denies involvement in the conflict that threatens to tear Ukraine apart and has dragged ties between Russia and the West to their lowest since the Cold War.
The Kiev leadership says Russian border authorities could do far more to stop these armed groups crossing into eastern Ukraine.
Violence in the east intensified in the run-up to the May 25 election in Ukraine in which a first round win by Poroshenko, a confectionery magnate, gave the signal for a Ukrainian military offensive against the separatists.
Foreign fighters at the Battalion Vostok base in the north-eastern part of Donetsk, a city of 1 million people, gave various reasons for joining the separatists defending the self-proclaimed "Donetsk People's Republic".
"My personal motivation is the religion, protecting Orthodox Christianity from the West," said Oleg, nicknamed Mamay, from North Ossetia, a Russian region on the northern rim of the Caucasus mountains.
Just south of his homeland is South Ossetia, a Russia-dependent breakaway region of Georgia, which is populated by the same, mostly Orthodox ethnic group as the northern part.
In 2008, Moscow and Tbilisi fought a brief war over South Ossetia, which Russia now considers an independent state, though it is unrecognised internationally.
Oleg - stout, black-haired and bearded, carrying a flak jacket and a short machine gun on his arm - said there were 15 Ossetians, including himself, fighting in the ranks of Battalion Vostok around the Donetsk airport on May 26-27.
He and other Vostok fighters said their casualties included one native of Chechnya, a region of Russia where Moscow fought two separatist wars and which is now run by a Kremlin appointee Ramzan Kadyrov, who runs his own militia, or "Kadyrovtsy".
"There are no Chechens now. There were. They left yesterday (on Thursday) with their injured and killed. There was only one casualty among the Chechens," Khodakovsky said. "They were volunteers, not Kadyrovtsy."
Echoing Moscow's denials of involvement, Kadyrov says he has not sent his men to fight in eastern Ukraine but that some have apparently gone of their own accord.
The separatist leader Denis Pushilin said after the battle for the airport, which is now controlled by the Ukrainian army, that the bodies of "volunteers" from Russia would be returned home, openly acknowledging involvement from across the border.
Battalion Vostok fighters also said their comrades included a few from the states of former Soviet Central Asia.
Khodakovsky said he had some 1,000 men in his unit now and that some more "volunteers" were coming, with experience of state security structures or the army.
The unit started gaining shape in early May as blood was drawn in the southeastern city ports of Odessa and Mariupol in clashes between pro-Russians and pro-Ukrainian protesters as well as the law enforcement bodies.
Until mid-March, Khodakovsky used to head the elite special unit "Alfa" in his native Donetsk region, a senior and prestigious role in the state security service.
He says his shift was gradual, from rejection of the interim authorities brought to power in Kiev by street protests that in February ousted the former president, Moscow ally Viktor Yanukovich, to an all-out rejection of being part of Ukraine.
Battalion Vostok is one of multiple armed groups fighting on the pro-Russian side in the east, with blurred command and coordination lines and different priorities.
The unit has shown signs recently of flexing its muscles and acting against some other separatists whom Battalion Vostok denounces as looters.
Khodakovsky, who is also responsible for state security in the separatist republic's "government", sides with another armed unit called Oplot and the separatist's "prime minister" Alexander Boroday, who publicly admits to being a Muscovite.
"Some Moscow politicians finance their own people in our government; that is an inevitable necessity," he said, without giving details.
He dismissed Pushilin, leader of the "Donetsk People's Republic", as just a figurehead and admitted he had no illusions as to Moscow's immediate goals in Ukraine's east.
"I think Russia uses us to pursue its geo-political interests, have a buffer between itself and the West. We do not deceive ourselves about that. But even knowing this, we stick to Russia because it is our culture," he said.
The rebels have now taken positions deeper into Donetsk, an industrial hub, setting up barricades and posts in residential areas in the hope that Ukraine's army would not fight in a densely populated area and endanger the urban infrastructure.
"We have no other option," Khodakovsky said. "They should understand the consequences of fighting within a city if Poroshenko wants to go down in history as 'The Bloody One'."
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