A group of Russian soldiers, who have been captured in eastern Ukraine, crossed the border "by accident", BBC quoted Russian military sources as saying Monday.
Ukraine, which said 10 Russian paratroopers have been captured, has released video interviews of some of the men.
The incident comes as the Ukrainian and Russian leaders hold talks in Belarus.
Over 2,000 people have died in months of fighting between Ukrainian forces and separatists in the Donetsk and Luhansk regions that declared independence from Kiev after Russia's annexation of the southern Crimean peninsula from Ukraine in March.