The revelation by the Organization for Security and Co-operation in Europe (OSCE) provides some of the strongest independent evidence to date of Russian President Vladimir Putin's direct involvement in the 13-month war in the neighbouring nation.
Kiev and its Western allies have long accused the Kremlin of covertly coordinating the loosely organised rebel units' tactics and backing them up with high-tech weapons and troops in their fight against Ukraine's pro-Western government.
The OSCE said the two wounded servicemen said in an interview conducted at Kiev's military hospital that they were armed when wounded and taken prisoner by Ukrainian government forces in the separatist eastern province of Lugansk on Saturday.
"Both individuals claimed that they were members of a unit of the Armed Forces of the Russian Federation. They claimed that they were on a reconnaissance mission. They were armed but had no orders to attack," the security body said in a report.
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There was no initial response to the findings from either the Kremlin or Russia's foreign ministry.
But initial state media coverage of the findings suggest that Moscow may try to either downplay or ignore the report.
Russia's TASS news agency misquoted the OSCE as saying that both Russians "claimed that they used to serve in a unit of the Russian Armed Forces."
Russian state TV aired an interview with Aleksandrov's wife on Wednesday saying that the 28-year-old professional soldier had quit his army reconnaissance unit in December.
Putin has described Russians discovered fighting in Ukraine as either "volunteers" or off-duty soldiers who crossed into the war zone out of patriotic pride and to take on the far-right extremists who Moscow claims are running Kiev.