The Ukrainian Security Service (SBU) says it has extracted a witness who can testify to Russia's military presence in the separatist-controlled parts of eastern Ukraine from the city of Sorokyne (formerly Krasnodon) in the Luhansk region and moved him into government-controlled territory.
The October 8 SBU statement said counterintelligence officers carried out a "carefully planned operation" after the alleged witness agreed that he would "voluntarily refuse to participate in the activities of illegally armed groups and leave the occupied territory of the Luhansk region."
Out of financial hardship, the man allegedly joined the Kremlin-backed separatists and worked on repairing damaged military hardware.
However, he said he realized "the mistaken steps he had taken, reaching out about his readiness to testify" about Russia's military presence in the Donbas.
The SBU said he had already provided investigators with "important evidence of Russia's armed aggression" in the country.
After reviewing the man's biography, the SBU determined he had not committed any crimes.
The Donbas conflict erupted in April 2014 when Russia-backed separatists started to forcibly take over government buildings and law enforcement agencies in the two easternmost regions of Donetsk and Luhansk.
The Kremlin has denied being a party to the conflict and says that any Russians fighting in the Donbas are "volunteers."
According to the UN, more than 13,000 people have been killed and nearly 2 million people displaced in the conflict.
Russia the same year sent troops into Ukraine's Crimean Peninsula, eventually taking it over, prompting Western countries to impose sanctions on Moscow.