Ukraine has said its forces had captured Russian paratroopers who had crossed into Ukrainian territory on a "special mission" - but Moscow said they had ended up there by mistake.
The Ukrainian military, meanwhile, reported pro-Russian separatist forces were shelling the town of Novoazovsk and Russian military hardware was building up over the border.
Earlier, the security services released video footage purporting to show testimonies from Russian paratroopers detained by Ukrainian government forces while fighting alongside pro-Moscow rebels in Ukraine.
The videos, the strongest evidence yet to back up Kiev's claims of Russian involvement in the conflict, overshadowed a meeting between Ukrainian President Petro Poroshenko and Russia's Vladimir Putinin the Belarussian capital Minsk.
Russia denies giving military help to the separatist rebellion.
The Russian defence ministry said the servicemen strayed into Ukrainian territory by mistake.
Ukraine rejected this. "It wasn't a mistake, but a special mission they were carrying out," military spokesman Andriy Lysenko said.
In footage posted on Kiev's Anti-Terrorist OperationFacebook page, one of the detained soldiers listed his paratroop regiment and said is based inthe Russian town of Kostroma.
Social networking pages appear to corroborate the men's assertions.