Russia says that a group Russian military personnel who were captured in eastern Ukraine had crossed the border by mistake.
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."
A Ukrainian television report that carried the interviews with the men said they were from the 331st regiment of the 98th Svirsk airborne division.
It quoted one man, named as Sgt Andrei Generalov, as saying: "Stop sending in our boys. Why? This is not our war. And if we weren't here, none of this would have happened."
Another man, named as Ivan Milchakov, says he is based in the Russian town of Kostroma.
"I did not see where we crossed the border. They just told us we were going on a 70km march over three days," he said.
"Everything is different here, not like they show it on television. We've come as cannon fodder."
Russia has repeatedly denied Ukrainian and Western accusations that it is supporting the rebels.