Russia has accused Ukraine of abducting two of its servicemen from the Crimean peninsula, which it annexed in 2014.
Ukraine-Russia-conflict: Moscow warns of attempt to destabilise region
The Russian defence ministry said the men were "illegally detained" on Sunday.
Ukraine says the pair is deserters, who defected to Russia from the Ukrainian military after Crimea was annexed in 2014.
Russia called their arrest "another act of gross provocation" and called for their "immediate return".
The men have been named as Ensign Maxim Evgenyevich Oditsov and Junior Sergeant Alexander Vyacheslavovich Baranov.
Russia said they were taken from Crimea to Ukraine's Mykolaiv region.