Vladimir Starkov is currently held in a detention center in Kiev after being intercepted three weeks ago with a truck full of ammunition.
He was driving the truck from Donetsk to Yasne, a small village south of the separatist stronghold of Donetsk when he was arrested.
Starkov, who is being charged with terrorism, explained to Euronews that he was recruited to help separatists in Eastern Ukraine.
He was in charge of weapon accountability.
“At the time I was detained, I was a regular serviceman in the armed forces of the Russian Federation with the rank of Major," Starkov told Euronews.
"I was chief of missile and artillery weapons service in the military unit in the Russian city of Novocherkassk."
He also told Ukrainian security services that about 2,000 Russian servicemen are deployed in Ukraine at the moment, and that like himself, many did not volunteer.
“Senior officers gathered us in a conference room and announced that our positions would be the same as promised, but we should do our military service in Ukraine: in the Donetsk and Luhansk People’s Republics," he said.
"It was forbidden to inform our relatives about this."
Once he was arrested, Starkov tried to call his family but couldn't get through to them.
The same happened when Euronews tried to call them.
The Russian Defense Ministry and Russian Embassy in Kiev both did not respond to Euronews’ request to comment on Starkov’s identity.
Although Russia has consistently denied sending servicemen to Ukraine, many reports contradicting their statements have surfaced.
The conflict with Russia in eastern Ukraine started in early 2014 following unrests in Kiev over policies that would have isolated Ukraine from the EU and brought it closer to Russia.
The unrests eventually led to the ousting of Ukraine's Kremlin-friendly puppet, then-President Viktor Yanukovych.
The fighting started escalating following the annexation of Crimea by Russia in March 2014.