A group of Russian soldiers captured in eastern Ukraine had crossed the border "by accident", Russian military sources arequoted as saying.
Ukraine said 10 paratroopers had been captured and has released video interviews of some of the men. One is quoted as saying "this is not our war".
The incident comes ahead of a key meeting between the Ukrainian and Russian presidents.
Petro Poroshenko and Vladimir Putin are at a summit in Minsk in Belarus.
More than 2,000 people have died in months of fighting between Ukrainian forces and separatists in the Donetsk and Luhansk regions.
The two regions declared independence from Kiev following Russia's annexation of the southern Crimean peninsula from Ukraine in March.