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.
A Russian defence ministry source was quoted by the Russian news agency RIA Novosti as saying: "The soldiers really did participate in a patrol of a section of the Russian-Ukrainian border, crossed it by accident on an unmarked section, and as far as we understand showed no resistance to the armed forces of Ukraine when they were detained."
Source (BBC): http://www.bbc.com/n...europe-28934213
Well... that's OK then - just an "accident"