The troops emerge and stagger into the middle of the dusty track, then onto their knees, their hands placed on their heads. Seconds later, the Ukrainian drone footage shows, they lie face down, motionless, dust trailing nearby.

Video obtained exclusively by CNN, filmed during fighting late August near the embattled city of Pokrovsk in eastern Ukraine, shows an apparent execution by Russian troops of three surrendering Ukrainians, after their trench was overrun.

The incident, described by a Ukrainian official who asked for some details to be withheld to protect the identity of the unit, is part of a pattern of apparent executions, seemingly increasing in pace this year.

Ukrainian defense intelligence sources have given CNN a list of 15 cases since November, most supported by drone video or audio intercepts, in which they say surrendering Ukrainian troops were killed by the Russians on the front lines, rather than being taken prisoner.