* "Russian Orthodox Army" is mostly Ukrainian but identifies as Russian
* Unit, formed 3 months ago, wear prayer ribbons and carry religious icons
* One of several armed rebel groups fighting in Ukraine's east
* Vows revenge after fire-fight with Ukrainian army over Donetsk airport
By Gabriela Baczynska
DONETSK, Ukraine, May 30 (Reuters) - They are tired after more than three days with almost no sleep but their eyes shine with battle fervour as they man the rebels' frontline barricade in Donetsk, the eastern Ukrainian city where dozens were killed in a battle over the airport this week.
"We are Russians, and we will take revenge for everything the Ukrainians have done to us. We will be here until the very end," said a 29-year-old native of the Sea of Azov port city of Mariupol, who only identified himself by the nickname Chrome.
Chrome and his 24 comrades on the barricade are part of a new pro-Moscow unit formed three months ago. Calling itself the "Russian Orthodox Army", it has been engaged in the heaviest fighting against Ukraine's army in the Donetsk region this month.
It is just one of several armed rebel groups fighting in Ukraine's tumultuous east, with blurred command and coordination lines - and varying motivations and allegiances.
Chrome previously worked on building sites and was an amateur diver and underwater hunter - two blue dolphins tattooed on the left side of his neck a reminder of that time.
Around his waist is wrapped a length of cloth on which prayers are written. Some of his comrades carry little Orthodox icons in the pockets of their mis-matched camouflage fatigues.
"There are many lines in the Bible. The New Testament speaks of turning the other cheek, but we will take it no more. We have the Old Testament here - an eye for an eye," said Sergei, a thin, black-haired sailor from Mariupol, who commands the last rebel barricade on the road from Donetsk to the airport.
But this is no religious crusade; theirs is primarily a patriotic zeal that cleaves to Russia, even though most were born in the Donetsk region.
Ukrainian army firepower pushed the separatists back from the airport earlier this week in a fierce assault in which about 50 rebels - many of them apparently from Russia - were killed.
The Russian Orthodox Army's barricade - made of sandbags, trucks and hunks of concrete - now sits between residential blocks in the city, significantly raising the risk of civilian casualties in the event of fighting.
Sergei said his men were also in control of the Ukrainian Security Service (SBU) building in Donetsk and that he was appointed commander by the separatists' top military figure Igor Strelkov, a Muscovite.
Strelkov has set up his main base in the town of Slaviansk, some 100 kilometres (62 miles) north of Donetsk, where the rebels on Thursday shot down a Ukrainian army helicopter, killing 14 soldiers.
Violence intensified in eastern Ukraine in the run-up to the May 25 presidential elections, which handed outright victory to confectionery billionaire Petro Poroshenko. Kiev stepped up its operation to crush the separatist rebellion following the vote.
In Donetsk, two days of heavy fighting on the outskirts of the city have alarmed its citizens and eroded the show of support for the rebels on the streets. But it seems to have only stiffened the resolve of the Russian Orthodox Army and other militia groups.
"We did not have any special training apart from the usual conscription service in the army. But none of us is afraid when we are in combat. We shoot to kill," Chrome said.
For all the rhetoric, the unit acknowledges it is short of ammunition and other supplies and relies on locals for food.
They fought in the fierce fire-fight with the Ukrainian army around the Donetsk airport on Monday and Tuesday and in the village of Karlovka last week when one of their men was killed.
In both cases they were aiding "Battalion Vostok" - or the "East Battalion" - the most heavily armed pro-Russian force that is now engaged in fighting in the Donetsk region.
Vostok is widely believed to have fighters in its ranks from Chechnya, a formerly rebel region of Russia that fought two separatist wars against Moscow and is now run by a Kremlin appointee Ramzan Kadyrov who runs his own pro-Moscow militia.
In Donetsk on Thursday rebels said the bodies of some "volunteers" from Russia, killed this week in the airport fighting, were being prepared to be returned to Russia - an acknowledgement of involvement by militia fighters from over the border.
Coffins were later loaded on to a vegetable truck at a Donetsk morgue and driven away.
Ukraine's interior minister, Arsen Avakov, has accused the government of Russian President Vladimir Putin of being behind the airport violence, which began when rebels seized a terminal on Monday.
Weapons collected at the airport after the rebels withdrew bore serial numbers and years of production that clearly indicated they had been brought in from Russia, he said.
Most men in the Russian Orthodox Army had never met before early March when, they say, 10 of them made contact online to deliver an appeal for help to Moscow via Russian envoys in Rostov, a Russian city 100 kms south-east of the Ukrainian border.
"Now one of our men is a coordinator for us in Moscow," Chrome said, refusing to give any more detail. (Additional reporting by Sabina Zawadzki in Donetsk; Editing by Richard Balmforth and Will Waterman)